In the year 1970, Prof. Niklaus Wirth invented the Pascal programming language as a way to teach his students the fundamentals of computer programming. Although the initial core Pascal language was designed for teaching purposes only, it was soon expanded by commercial vendors and gained some popularity. Later, Wirth presented the language Modula-2 with improved syntax and the module concept for larger projects, and the Oberon language family with additional support for Object-Oriented Programming.

The Nim programming language can be seen in this tradition, as it is basically an easy language suited for beginners with no prior programming experience, but at the same time is not restricted in any way. Nim offers all the concepts of modern and powerful programming languages, combined with high performance and a certain level of universality. Nim can be used to create programs for tiny microcontrollers, large desktop apps, and web applications. Most books about programming languages focus on the language itself, often assuming that the reader is already familiar with the foundations of computer hardware and has some programming experience. This is generally a valid approach, as most people are taught this fundamental knowledge, sometimes referred to as Computer Science (CS), in school today. However, there are people who, for various reasons, may have missed this introduction in school and later decide that they need some programming skills, perhaps for a technical job. Moreover, some children may not be satisfied with the introduction to computer science taught at school. Therefore, we decided to start this book with a short introduction to fundamental concepts. Most people may skip that part, but you should be really sure that you know these foundations. This book is divided into seven parts — part VII is the Appendix. It is possible to read the parts independently of each other in any order, but for Nim beginners, it is recommended to read them mostly in ascending order, perhaps while previewing some interesting sections in the second half of the book early on. In Part II, we explain the basics of computer programming step by step in a way that should enable even those with no prior experience to learn independently. In this part, we might repeat some of the material that we already mentioned in Part I. We do that intentionally, as some people might skip Part I, and because it is generally beneficial to reinforce the reader’s learning process through repetition. Part III will give you an overview of Nim’s standard library, which contains many useful functions and data types that we can use in our programs to solve common tasks like input and output operations, using the file system, or sorting data. In Part IV, we will apply what we have learned by solving some common programming tasks, like sorting, searching, or converting numbers from the internal computer format to displayable text. Part V will introduce some useful external packages that can be easily installed using one of Nim’s package managers. Nim already has a few thousand external packages — some of them may support or replace the standard library, and others offer special or advanced functionalities. Part VI of the book will finally introduce advanced concepts like asynchronous operations, threading and parallel processing, macros and meta-programming, and, last but not least, Nim’s concept implementation. Some sections, that do not integrate well into the other six parts, or that are boring or useful only for a minority of Nim users, have been moved to the Appendix and may not be part of a printed copy of the book. This currently includes a short introduction to Nim’s standard package manager: Nimble.

This book is essentially a traditional textbook — simple yet detailed. It is designed such that individuals aged 14 and above can read and understand it independently, with little or no help from adults. Unfortunately, the English language may still be a challenge for many kids not born in a country with a strong English language tradition. Fortunately, automatic translations are already supported for some languages, and we might be able to offer translated editions of the book later, possibly in Chinese and German.

In the last few decades in the area of computer programming, traditional textbooks have been partly replaced by videos, "Crash course" books, and "Learning by doing" books. Indeed, a good video may help you start with a new language, and it can enable people who have difficulties reading printed texts or concentrating on a topic for a few minutes to learn a programming language. Unfortunately, the quality of most videos is very bad; some are made by kids just having learned the first steps of computer programming themselves. Furthermore, watching videos does not necessarily improve the reading and concentration issues that people might have. "Crash course" and "Learning by doing" books may give you a good start, but for that, we already have a lot of textual tutorials. The concern with these types of books is that, while they may help you solve common tasks, they don’t necessarily foster a deeper understanding. Generally, the idea of a "Crash course" or "Learning by doing" is not bad. However, in computer science, starting with a larger example application can be overwhelming, as you have to learn a lot of things simultaneously. It may work for you, but there is the danger that you forget all the details very quickly again. Moreover, these types of books are not very helpful when you need to look something up. The other concern with "Learning by doing" in computer science is that learning materials may have only examples in which you may not be really interested: Of course, we can create a simple chat application, a simple Twitter clone, and do some basic web scraping using async/await. Or create a basic game or a simple GUI with one of the dozen available toolkits. But what if you are not interested in chatting and twittering, and that single selected toolkit? We believe that in such cases, reading the detailed examples can be very frustrating. Therefore, we recommend that after reading the first tutorial, and perhaps a few pages of this book, you start coding with topics you are interested in. Perhaps you could do it together with some friends? Whenever you need concrete help, you can find it on the Internet, using search engines, Wikipedia, or a discussion platform of your choice. And if you really have no idea what project to start with, then computer programming might not be the right profession for you.

Although Nim has a JavaScript backend and thus well supports web-related development, this book focuses on native code generation using the C and C++ backends. We will discuss some peculiarities of the JavaScript backend in the second half of the book, and we may provide some examples of the use of the JavaScript backend in the Appendix. If you are strongly interested in web development and the JavaScript backend, then you may also consult the book Nim in Action by Dominik Picheta, which gives some detailed examples for the development of web-based software using the Nim programming language, including a simple chat application and the foundation of a microblogging and social networking service. You may also consult the tutorials and manuals of Nim web packages like Karax, Jester, or Basolato.

This book will not attempt to explain things that are already well-explained elsewhere, or that should have been well-explained elsewhere — at least not in this first edition, where we have many other essential topics to cover. So, for now, we will leave out the following: the installation of the compiler, the process of installing and using text editors or IDEs with special Nim support, the use of Nim package managers such as Nimble and Nimph, the use of the foreign function interface (FFI) to create bindings to C libraries, and internal compiler details like the various memory management options and all the pragmas.^[1] Also, we do not intend to fill the book with redundant information, such as tables listing all the Nim keywords or Nim’s primitive data types, as you can easily find all of that in the Nim language manual.

While the creation of graphical user interfaces (GUIs) is an important topic, we cannot provide many details for various reasons. Nim does not have a singularly accepted GUI library, but there are more than 20 attempts — from pure Nim ones like NimX or Fidget, to wrapped libraries like GTK or QML, to GUIs that try to provide a native look for various operating systems like XWidgets or NiGui, and even web-based GUIs. And for each of these, at least for the more serious ones, we could write a separate GUI book. Therefore, we will only provide a few minimal examples for some of them in Parts IV or V of the book.

Furthermore, we will not delve into game programming, as it is a broad area with numerous existing tutorials.

Maybe in later editions of the book, we will add some more topics, e.g. game programming, as so many people like it. However, we will always have to ensure that a potential printed version of the book does not exceed 500 pages, which may require us to exclude some content in the printed version.

Generally, when learning a new programming language, people start with some short tutorials before delving deeper into the language by following a book. This approach is indeed a good start. So we recommend that you read the short official tutorials, parts 1 and 2, and perhaps also some other tutorials freely available online. Tutorials typically only scratch the surface of the topics, so you may not fully understand them all, but this approach gives you a feel for the language. There also exist some video tutorials, in case you have problems reading. However, if that’s the case, this book might not be of much use to you. If you already have a background in computer science and experience with other languages such as C++, Haskell, or Rust, the tutorials and the Nim language manual might be fully sufficient for you; thus, you may not need this book at all. Or you may prefer the recently published book of Mr. Rumpf, called "Mastering Nim: A complete guide to the programming language" available at Amazon.com.

This book is based on the Nim reference implementation by Mr. A. Rumpf. Most explanations and examples should also be valid for other implementations, like the one at https://github.com/nim-works/nimskull.

Although the initial pages of this book were written in the spring of 2020, the book should be mostly up-to-date with Nim versions 1.6 and 2.0.

Nim version 1.6.14 was released on 27 June 2023 and includes many bug fixes for the 1.0 branch. Nim 2.0, initially announced for early 2023, was finally released on 01 August 2023.

The v2.0 release brings many improvements but does not include any serious breaking changes that would invalidate old code. Only a few minor modifications might be necessary for old code to compile and run again. In this book, we may use and discuss a few Nim 2.0 features, but most code should be compatible with the 1.x series of the compiler or with nimskull (cyo) with no or only minor changes.

The most significant change in Nim 2.0 is that ORC memory management has become the default indicating that it is considered ready for use in production. ORC gives us GC-like, fully deterministic memory management with minimal overhead compared to manual memory handling. It reduces the maximal memory consumption of apps, avoids GC-generated delays, and may increase the performance of our programs. Additionally, ARC and ORC memory management should bring serious advantages for the creation and performance of threaded and parallel code. We have summarized the most important new features of Nim 2.0 in the appendix titled Changes for Nim 2.0.

Note that incremental compilation (IC) or CPS task scheduling (Continuation-passing style) is still in development and not yet fully supported by Nim v2.0. And for parallel and threaded code execution, it may be useful to consider high-quality external libraries rather than those in Nim’s standard library.^[2]. This may also apply to modules for asynchronous code execution and a few other libraries.^[3]

For all the details please refer to the corresponding section in the Appendix: Disclaimer and legal notice

Since spring 2023, GPT-4 has been available to paying subscribers, offering more helpful information and less irrelevant content.

We recently used GPT-4 to create a short Nim info page: https://nimprogramming.com/

A computer is primarily a device that runs computer programs by following instructions on how to manipulate data.

Nearly all computers currently in use — from tiny ones integrated into electronic gadgets, to well-known desktop computers (PCs), and large, powerful supercomputers filling entire rooms — work internally with digital data only.^[8] Digital data essentially comprises integer (whole) numbers encoded in binary form, which are represented by sequences of the symbols 0 and 1. We will discuss the term digital in more detail in the next section.

The most important part of a digital computer is the CPU, the Central Processing Unit. This tiny device, built of digital electronic circuits, can perform very basic mathematical and logical operations on numbers, such as adding two numbers or determining whether one number is larger or smaller than another. Most computer CPUs can only store a limited number of values internally, which are lost when the power is switched off. Therefore, the CPU is typically electrically connected to a RAM module, a Random Access Memory, which can store many more numbers and allows fast access to these numbers, and to a hard disk or SSD device, which can permanently store the numbers but does not allow such fast access. The stored numbers are most often simply referred to as data; in essence, this data is nothing more than numbers, but it can be interpreted in various ways, such as pictures, sounds, and more.

The traditional hard disk drives (HDDs), which store data electromechanically on rotating magnetic disks, as well as the more modern variants, the solid-state devices (SSDs), which store data using modern semiconductor technologies, can store data persistently for longer time periods, even when no electric power supply is available. Both SSDs and HDDs can be optionally split into multiple partitions; for example, one or multiple OS partitions for executable programs or pure data partitions for passive data such as text files or pictures. Before use, each partition is generally formatted, at which point a file system (FS) is created. These two steps create an internal structure on the storage device, which allows us to store and retrieve individual data blocks like programs, text files, or pictures.

Nearly all of today’s desktop computers, as well as most notebooks and cellphones, contain not just a single CPU, but multiple CPUs, also known as cores. This enables them to run different programs in parallel or parts of a single program on different CPUs to increase performance and reduce total execution time. So-called supercomputers can contain thousands of CPUs. Besides CPUs, most computers also have at least one GPU, a Graphic Processing Unit, that can be used to display data on a screen or monitor, maybe for doing animations in games or for playing video. The distinction between CPUs and GPUs is not clear-cut. Usually, a CPU can also display data on screens and monitors, and GPUs can also perform some data processing tasks that CPUs can handle. However, GPUs are optimized for the data display task.

More visible to the average computer user are the peripheral devices such as a keyboard, mouse, screen, and perhaps a printer. These enable human interaction with the computer, but they are not core components — the computer can function effectively without them. In notebooks, laptop computers, or cell phones, the peripheral devices are closely integrated with the core components. All the physical parts of a computer are also called hardware, while the programs running on that hardware are called software.

A less visible, but equally important, class of computers consists of microcontrollers and so-called embedded devices. These are typically tiny units encased in black plastic with some electrical contacts. The devices can contain all necessary elements, i.e., the CPU, some RAM, and persistent storage that can store programs and data when no electric power supply is available. Although these devices may be limited in computing power and the amount of data they can store and process, they are incorporated in many consumer devices. They control your washing machine, refrigerator, television, radio, and others. Some devices in your home may even contain multiple microcontrollers and often the microcontrollers can already communicate with each other by RF (Radio-Frequency), or access the Internet by WLAN, which is sometimes called the Internet of Things (IoT).

Another class of large, very powerful digital computers — known as mainframe computers or supercomputers — is optimized to process large amounts of data very quickly. The key to their enormous computing power lies in many fast CPUs working in parallel; problems or tasks are split into many small parts that are solved by individual CPUs, and the final result is the combination of all these solved sub-tasks. However, it is not always possible to split large problems into smaller sub-tasks.

Digital computers usually operate based on a clock signal that pulses at a certain frequency; the number of clock pulses per second is called the clock rate. The CPU can perform simple operations, such as the addition of two integers, at each pulse of the clock signal. For more complicated operations, such as multiplication or division, it may need more clock pulses. Therefore, a rough measure of a computer’s performance is the clock rate divided by the number of pulses that the CPU needs to perform a basic operation, multiplied by the number of CPUs or cores that the computer can use.

A completely different type of computer is the quantum computer. This is a large, expensive high-tech device that uses the principles of quantum mechanics to execute many computations simultaneously. Only a few of them exist today, for research at universities and some large commercial institutes. Quantum computers may fundamentally change computing and our entire world someday, but they are not the topic of this book.

Whenever we measure a quantity using a base unit, thus providing a certain level of granularity, we operate within the digital realm. Our ordinary money is digital, as the cent is the smallest base unit; you will never pay a fraction of a cent for something. Time can be considered as a digital quantity as long as we accept the second as the smallest unit. Even on so-called analogue watches, the second hand generally moves forward in one-second increments, making it impossible to measure fractions of a second with such a watch.

An obvious analogue property is the thermodynamic temperature, and its classic measurement device is the well-known capillary thermometer, consisting of a glass capillary filled with alcohol or liquid mercury. When temperature increases, the liquid in a reservoir expands more than the surrounding glass and partly fills the capillary. That filling rate is an analogue measure of the temperature.

While the hourglass is considered digital (as you can count the tiny sand grains), the sundial is not.

Most quantities in the real world appear to be analogue, and digital quantities are often perceived as an arbitrary approximation. However, quantum mechanics has taught us that many quantities in our world do have a granularity. In physical terms, quantities such as energy or momentum are multiples of the tiny Planck constant. Or consider electric charge, which is always a multiple of the elementary charge unit of a single electron. Whenever electrical current flows through a conductor such as a wire, an ionized gas, or an electrolyte like saltwater, it does so in multiples of the elementary charge, not in fractions of it. And of course, light and electromagnetic radiation also have some form of granularity, which the photoelectric effect, as well as Compton scattering, proves.

An important and useful feature of digital signals and data is their direct correlation to integers (integral numbers).

The simplest form of digital data is binary data, which can only have two distinct values. When you use a mechanical switch to turn the light bulb in your house on or off, you change the binary state of the light. Your neighbor, when watching your house, receives binary signals.^[9]

Digital computers generally use binary electric states internally — voltage or current on or off. Such an on/off state is called a bit. We will discuss more details about bits and binary logic later. One bit can obviously store only two states, which we may map to the numbers 0 and 1. Larger integer numbers can be represented by a sequence of multiple bits.

The Morse code was an early application used to transmit messages encoded in binary form.

A crucial characteristic of digitally encoded data is its ability to be copied and transmitted without loss of precision. The reason for this is that digital numbers have a well-defined clean state, there is no noise overlaying the data that could accumulate when the data is copied multiple times. Well, this statement isn’t entirely accurate — under poor conditions, noise can become significant enough to alter the binary state of signals. Imagine trying to transfer some whole numbers encoded in binary form, perhaps by binary states represented as voltage levels 0 Volt and 5 Volts, over an electric wire across a long distance. Clearly, the long wire can act as an antenna and pick up electromagnetic noise, which could potentially shift the true 0 Volt data to a voltage closer to 5 Volts, leading to incorrect reception. To detect these types of transmission errors, checksums are added to the actual data. A checksum, derived from the original data using a special mathematical formula, is transferred with it. The receiver applies the same formula to the received data and compares the result with the received checksum. If they do not match, it is clear that the data transmission is corrupted, and a resend is requested.

The opposite of digital is generally called analogue, a term that is used for data that has or seems to have no granularity. For example, we speak of an analogue voltage when the voltage can assume any value in a given range, and when it does not "jump" but changes continuously.^[10] To observe analogue voltages or currents, one can use a moving coil meter, a device in which the current flowing through a coil in a magnetic field causes the magnetic force to move the hand/pointer.

As mentioned in the previous section, nearly all of our current computers work exclusively with digital data. Essentially, this means they work internally with integer numbers, stored in sequences of binary bits. All input for computers must have the form of integer numbers and all output takes the form of integer numbers. Whenever we need to input analogue data into computers, such as analogue voltage, we must convert it into a digital approximation. For that task, special devices called analogue to digital converters (ADC) exist. And in some cases, we have to convert the digital output data of computers to analogue signals, like when a computer plays music: The digital data output from the computer is then converted by a device known as a digital to analogue converter (DAC) into an analogue voltage. This analogue voltage generates a current that flows through a coil in our speakers. This electric current in turn generates a magnetic field, which exerts mechanical forces that move the speaker’s membrane. The resulting oscillating movements produce variations in air pressure that our ears detect, and that we perceive as sound.

Most computers, from cellphones to large supercomputers, use an operating system (OS). A well-known OS is GNU/Linux. An operating system can be seen as the initial program that is loaded and started when we switch the computer on, functioning as a kind of supervisor:^[11] it can load and execute other programs, distributing resources like CPU cores or RAM among multiple running programs. It also manages user input via the keyboard and mouse, displays output data on the screen in both textual and graphically forms, controls how data is stored in nonvolatile storage media like hard disks or SSDs, oversees all network traffic, among other tasks. An important role of the OS is enabling user programs to access all the various hardware components, regardless of vendor, in a uniform, high-level manner. An OS can be seen as an intermediary layer between user programs, such as a text processor or a game, and the computer’s hardware. The OS allows user programs to work on a higher level of abstraction, so they do not need to know much about the low-level hardware details.

An important feature of most modern operating systems is their ability to run multiple system and user programs concurrently or in parallel. Concurrent execution of programs means that the execution swiftly switches between all active programs. In this way, the user does not notice when programs pause for short time intervals. All programs appear to be running continuously, though not necessarily at full speed. True parallel execution of programs, meaning they can all run continuously at full speed, is only possible when the computer has multiple CPUs or a CPU with multiple physical cores.

Computer operating systems generally have a close relationship with software libraries. Libraries are software components that provide data types and functions through a well-defined interface, known as an Application Programming Interface (API), and exhibit specific behaviors. Libraries can either be part of the OS, or they can function largely independently of it.

Libraries can be utilized as shared libraries, which are single binary files stored on a computer’s file system — often with the .so or .dll file extension — and are accessible by different computer programs simultaneously. They can also be used as static libraries, which are an integral part of individual programs. Shared libraries have some advantages: we need only one instance of them on the file system of the computer, and the library is loaded only once into the computer memory (RAM), even when it is used by different apps simultaneously. This saves space, and when the library has serious errors, it is in principle possible to replace the library with a corrected version, which is then used by all the software on the computer. Shared libraries often come in numbered versions, where a higher number denotes a newer, improved, or extended library version. Sometimes, some of the programs we use may still need an older library version, while other software already needs a newer one. In that case, our file system has to provide multiple versions of a shared library, each of which can be used independently. On the other hand, statically linked libraries are directly glued with a single computer program. This simplifies the distribution of the program, as it can be shipped as a single entity without the need to ensure that all the necessary dynamic libraries are available on the destination computer. However, if a statically linked library has serious errors, then we have to replace all the programs that are linked statically with that corrupted library.

Small microcontrollers and embedded devices often do not require an operating system as they generally run only one single-user program and typically lack a wide variety of hardware components for support.

To interact with the OS and the application programs running on the computer, we need some form of user interface. Traditional user interfaces are text-centric and often provided directly by the OS as one single text screen filling the whole display: The user has to enter textual commands and the computer reacts with textual messages. For entering commands and data, a keyboard, whose layout was heavily inspired by the classical mechanical typewriter, is used. For about half a century now, graphical user interfaces (GUIs) have mostly replaced, or at least supplemented, textual user interfaces for desktop computers. Even cellphones and other electronic gadgets now use a form of GUI for user interaction. For large mainframe computers, the textual user interface is still common. Graphical user interfaces display sets of icons or widgets to the user. These are often arranged within rectangular graphical boxes, known as windows. These windows can be moved around, resized, and partially or fully overlapped with other windows. A special type of window, known as a terminal, shell, or console window, behaves like the traditional full-screen textual user interfaces. Graphical user interfaces allow users to interact with the computer through simple actions like clicking on buttons or using drag or swipe gestures, performed directly on a touch-sensitive display or with a device called a mouse, which mirrors its mechanical movement on the table to a graphical cursor on the computer display, and provides a set of pushbuttons that are used to initiate a click action when the mouse pointer hovers over an icon or widget. The main advantage of graphical user interfaces is that the user does not have to remember and type in long command sequences. A set of on-screen buttons labeled with single letters can simulate a traditional keyboard, but a physical keyboard is still used when the input of longer textual data is required. Graphical user interfaces are sometimes enhanced by speech recognition systems, which allow users to enter commands or textual messages vocally. Graphical user interfaces may appear to be strongly coupled with the OS, but they are still system programs executed by the OS. For the Microsoft Windows OS and the macOS, this distinction is not very obvious, as the same GUI is running permanently. For other operating systems, like Linux, the distinction is more apparent. Linux systems are sometimes used without a GUI, and various GUI toolkits, such as Gnome, KDE, and many others, are available.

Computer programming involves the creation, testing, and optimization of computer programs.

A computer program is essentially a sequence of numbers that are meaningful to a computer CPU. The CPU recognizes these numbers as instructions or numeric machine code, such as the instruction to add two numbers. The first computers, built in the 1950s, were programmed by feeding sequences of plain numbers to the device. The numbers were stored on what were known as punch cards. These were made of strong paper and the numbers were encoded through holes in the cards. The holes could be recognized by electrical contacts to feed the numbers into the CPU. Since plain numbers do not align well with human cognition, more abstract codes were soon developed and used. A very direct code that matches numerical instructions to symbols is known as the assembly language. In that language, for example, the character sequence "add A0, $8" may map directly to a sequence of numbers which instructs the CPU to add the constant integer number 8 to CPU register A0, where A0 is a storage area in the CPU where numbers can be stored. As many different types of CPUs exist, each with their own instruction sets, there are also many different assembly instruction sets. These have similar, but not identical instructions. The rules that describe how these basic instructions have to look are called the syntax of the assembly language.

Numerical machine code, and its equivalent assembly language, form the most basic instruction set for a CPU. Each command that a CPU can execute corresponds to a well-defined assembly instruction. Thus, any operation that a computer can potentially execute can be represented as a series of assembly instructions. However, complicated tasks may require millions of assembly instructions, which would take humans a significant amount of time to write, modify, proofread, and debug.^[12]

A few years after the invention of the first computers, the need for more abstract instruction sets was recognized. These would include features such as repeated execution, composed conditionals, and the ability to use data types beyond plain numbers as operands. As a result, higher-level programming languages such as Algol, Fortran, C, Pascal, and Basic were created.

An algorithm is a detailed sequence of instructions, often abstract, designed to solve a specific task or to reach a goal.

Recipes from cookbooks and car repair instructions are examples of algorithms. The basic math operations children learn in school, such as adding, multiplying, or dividing two numbers with a paper and pencil, are also examples of algorithms. Even starting a car follows an algorithm. For instance, if the temperature is below freezing and your vehicle is covered in snow, your first step would be to clean the windows and lights. Similarly, if you’re driving again after a long break, you would have to check the tires before you start the engine. You can execute an algorithm by strictly following its instructions, without necessarily understanding its underlying principles.

So an algorithm is a perfect fit for a computer, as computers are excellent at following instructions without really understanding what they are trying to accomplish.

An algorithm for calculating the sum of the first 100 natural numbers might look like this:

Most traditional programming languages were designed to translate algorithms into elementary CPU instructions. Algorithms typically contain nested conditionals, repetition, math operations, recovery from errors, and potentially plausibility checks. A complex algorithm can generally be split into various separate logical parts. These may include reading in data at one point, performing multiple processing steps at another, and storing or displaying data as plain text, graphics, or animation at yet another point. This division into parts is reflected in programming languages through the grouping of tasks into subroutines, functions, or procedures, which accept a set of input parameters and can return a result.

As algorithms often work not only with numbers but also with text, it makes sense to have a form of textual data type in a programming language too. Data types can also be grouped in various ways. For example, as sequences of multiple data of the same type, like lists of numbers or names. Alternatively, collections of different types can be created, such as the name, age, and profession of a citizen in an income tax database. Programming languages provide support for all these use cases.

We already learned that the CPU in the computer can execute only simple instructions, which we call numeric machine code or assembly instructions.

To run a program written in a high-level language that includes many abstractions, we need some kind of converter to transform that program into the basic instructions that the CPU can execute. For the conversion process, we essentially have two options: we can either convert the entire program into machine code, store it on disk, and then run it on the CPU, or we can convert it in small portions, maybe line by line, and run each portion as soon as we have converted it. Tools that convert the whole program first are called compilers. Compilers process the program that we have written, incorporate necessary library modules from other sources, check the code for obvious errors, and then generate the machine code, which we can then store and run. Typically compilers create executables that are customized for a specific CPU architecture and a single operation system. A program compiled for a x86 CPU and the Windows OS could not be run on a Linux box with an ARM CPU. Often, recompiling the source code for another target architecture is possible, but modifications to the source code may be necessary. Program code that has to be compiled can be distributes as textual source code, or as precompiled binary. For source code distribution, the targets systems needs a matching compiler, and for binary distribution, the binary has to match the CPU and the OS of the target system.

Tools that process the source code in small portions, like single statements, are called interpreters. They read a line of source code, investigate it to check if it is a valid statement, and then feed the CPU with corresponding instructions to execute it. The difference between compilers and interpreters is similar to two methods of picking strawberries: you can either pick one and eat it immediately, or you can collect them all into a basket to eat later. Interpreted program code is typically distributed as textual source code and can in principle be run on each system with an matching interpreter. But in practice, it is not that easy: The code may use functionality that is only available for a specific OS, or the code may require a specific interpreter version.

Both interpreters and compilers have advantages and disadvantages for special use cases. Compilers are capable of detecting errors before the program is run, and compiled programs generally execute quickly, as all the instructions are preprocessed and readily available when the programs run. The compiling step takes some time, of course, at least a few seconds, but for some languages and large programs, it may take much longer. This can slow down the software development process because, as you add or change code, you must compile the whole program before you can execute and test it. That can be inconvenient for beginner programmers, as they may have to do this editing and testing process very often. Some adopt a programming style that involves changing a tiny bit of the source code, running it, and observing the results. A more common practice, however, is to first thoroughly consider the problem, then write the code which, in most cases, performs nearly as intended. With this style of programming, you don’t need to compile and execute your code as frequently. Compilers have one significant benefit: they can detect many bugs, primarily typing errors, during the compilation phase and provide detailed error messages. Interpreters have the advantage of enabling code modifications and immediate execution without any delay. This feature is beneficial for learning a new language and for conducting quick tests; however, even simple typing errors can only be detected when encountered during program execution. If your test does not attempt to run a faulty statement, there will be no error, but it may surface later. Modern compilers use various techniques to enable also nearly immediate test when a part of the source code has been modified: Fast compilers, often running in parallel on all available CPUs, combined with caching and incremental compilation, makes the compilation step extremely fast. Additional, a technique called hot code reloading enables the exchange of parts of the program code without interrupting the program execution.

Generally, the execution of interpreted programs is much slower than that of compiled executables, as the interpreter has to continually process the source code in real-time as it’s being run, while the compiler does it only once before the program is run. To conclude this section, here are a few additional notes:

Compilers are sometimes paired with entities known as linkers. In such instances, the compiler transforms the source code, which may reside in multiple text files, into a sequence of machine code instructions. Subsequently, the linker amalgamates all these machine code instructions to form the final executable. Some compilers either do not require the linking step or automatically invoke the linker. Moreover, some interpreters convert the textual source code into so-called bytecode in a very fast, initial preprocessing step ("on the fly"), which can then be interpreted faster. Languages such as Ruby and Python employ this method. The Java language uses a mix of compilation and interpretation: In a first step, the Java source code is compiled into an intermediate JAR code format. This JAR file can be distributed and executed by Java’s virtual machine (JVM). The JVM acts as an intermediate layer between the hardware and the user program, and the JVM can even further optimize the code while it is run on the target machine.

Software can be crafted in numerous styles. A programming paradigm is a fundamental style of writing software, and each programming language supports a specific set of these paradigms. A popular paradigm is object-oriented programming (OOP), a concept taught in many introductory computer science courses. Other paradigm are procedural and functional programming.

We have already mentioned assembly languages, which provide only the basic operations that a CPU can perform. Assembly languages offer no abstractions, so it’s debatable whether we should categorize them as programming languages at all. Then, there are low-level languages like Fortran or C, which, while providing some basic abstractions, still work close to the hardware. These languages are primarily designed for high performance and low resource consumption (RAM), but they don’t prioritize detecting and preventing programming errors or simplifying the programming process. These languages already support some higher-order data types, like floating-point numbers or text (strings), as well as homogeneous, fixed-size containers (called arrays in C), and heterogeneous fixed-size containers (called structs in C).

A different approach is taken by languages like Python or Ruby, which aim to make writing code easier by offering many high-level abstractions. They provide better protection against errors but are not as efficient. These languages also support dynamic containers, which can grow and shrink, or advanced data structures like hash tables (maps) or support textual pattern matching by regular expressions (regex).

Another way to differentiate programming languages is by their typing system, which can either be static or dynamic. Ruby, Python, and JavaScript are all examples of dynamically typed languages. This means that they use variables capable of storing any data type. Therefore, the data type that a variable accepts can dynamically change during program execution. This appears to be user-friendly and often it is, particularly for brief programs intended for single-use, occasionally referred to as scripts. However, dynamic typing can make discovering logical errors more challenging. For instance, an illegal addition of a number to a letter may only be detected at runtime. Dynamically typed languages generally consume a lot of memory and their performance tends not to be as efficient. It’s akin to owning a set of large, equally-sized moving boxes and storing each piece of our belongings in separate boxes.

In statically typed languages, each variable has a well-defined data type such as integer number, real number, a single letter, a text element, and many more. The data type is either assigned by the author of the program with a type declaration, or is detected by the compiler itself when processing the program source code, a process called type inference. In this context, the variable’s type never changes. In this way, the compiler can check for logical errors early in the compile process, and the compiler can reserve memory blocks exactly customized to the variables that we want to store, so total memory consumption and performance can be optimized. Referring again to the box analogy, static typing is akin to using customized boxes for all your belongings.

All these types of programming languages are often called imperative programming languages, as the program specifies exactly what the computer has to do. There are also other types of programming languages, such as Prolog, which primarily provide a set of rules and then allow the computer to solve problems using these rules.

Moreover, there are emerging concepts like artificial intelligence (AI) and machine learning (ML). They rely less on algorithms and more on neural networks, which are trained with extensive data until they can yield the desired results. Nim, the computer language that this book focuses on, is an imperative language. As such, our focus will be on the imperative programming style. However, it’s worth noting that Nim can be used to create AI applications.

Additionally, we can distinguish between languages such as C, C++, Ada, Rust, D, Go, Nim, and many more that compile to native executables and can run directly on the computer’s hardware. In contrast, languages like Java, Scala, Kotlin, Julia, among others, use a large virtual machine (VM) as an intermediary between the program and the hardware, as do interpreted languages like Ruby and Python. Languages that use a virtual machine generally require some startup time when a program is invoked, as the VM needs to be loaded and initialized. Also, interpreted languages are typically slower.^[13] The distinction between languages that compile to native executables, and those that are executed on a virtual machine, is not really sharp. For instance, Kotlin and Julia initially ran on a virtual machine, but they can now compile source code to native executables. And new developments, such as the Mojo languages, claims to be able to execute ordinary Python code, as well as to compile code with added type annotations to fast machine code.

An important class of programming languages is the group of so-called Object-Oriented-Programming (OOP) languages, which use classes with attached methods, and typically reference semantics, polymorphism, and inheritance with dynamic dispatch. OOP languages became very popular in the 1990s. For some time, it was assumed that Object-Oriented-Programming was the ultimate solution for managing and structuring large programs. Java is a prominent example of OOP languages. It requires programmers to use the OOP design, and other languages such as C++, Python, and Ruby also strongly encourage the use of the OOP design. Experience has shown that the OOP design is not the ultimate solution for all computing problems, as it can make the code verbose and might hinder optimal performance. So newer languages, like Go, Rust, and Nim, support some form of OOP programming but use it only as one paradigm among many others.

Another popular and important class of programming languages includes JavaScript and its more modern extensions, like TypeScript, among others. JavaScript was designed to run in web browsers to support interactive web pages, as well as programs and games running in the browser. In this way, programs become nearly independent of the computer’s native operating system. Note that despite what the name may suggest, JavaScript is not closely related to the Java language. Since Nim can compile to a JavaScript backend, it offers robust support for web development.

Finally, perhaps the most important criterion for choosing a language for a programming task is the handling of memory and other resources. Allocating memory blocks, and releasing them again when they are not needed anymore, can be a serious effort, and doing it wrong can lead to various bugs, like free-after-use or memory leaks. The original Pascal compiler had no function to release memory at all, which may have been a simple strategy to avoid this difficult matter. C does all the memory- and resource-handling manually, which is one reason why C programming is difficult, and C programs often have serious bugs. The C++ language handles most memory and resource management by scope-based destructors, but still supports manual memory- and resource handling like C. Rust is similar to C++ in this regard, but has advanced features like the borrow-checker. Fully automatic memory management is a difficult topic and can generate overhead or delay in program execution. This is why some modern languages, like Zig, Odin, and Jai, avoid automatic memory handling. Other languages like Python, Java, JavaScript, C#, Julia, Go, and D use some form of garbage collector, which makes life for the programmers much easier and avoids all the memory-management-related bugs.

Nim was initially designed to use a garbage collector, with an option for manual memory management in critical areas. However, since version 1.0, Nim additional supports ORC/ARC memory handling, a form of scope- and destructor-based automatic memory management. ARC can be used when our memory blocks have no cycles, which is often the case. And ORC can handle additional cyclic structures. ARC and ORC may not yet provide optimal throughput compared to the older Garbage Collector, referred to as REFC. However, they avoid delayed deallocation and delays in program execution, making them good choices for critical code like device drivers and games.

Three well-known traditional programming languages are C, Java, and Python. C, created in 1972, is essentially a simple language that operates close to the hardware. Compilers can generate fast, highly optimized native machine code for C. However, C has cryptic syntax, some peculiar semantics, and it lacks the higher concepts of modern languages. Java, created in 1995, strongly encourages the object-oriented style of programming (OOP) and runs on a virtual machine. This makes it unsuitable for embedded systems and microcontrollers. Python, created in 1991, is generally an interpreted language rather than a compiled one, which results in slower program execution. Both, Java and Python, do not effectively support writing of low-level code that operates close to the hardware, making them unusable for device-driver and kernel development. Because many Python libraries are written in highly optimized C, Python can appear quite fast when performing standard tasks, such as sorting data, processing CSV or JSON files, or crawling websites. Therefore, Python is not a poor choice when primarily used for calling library functions. However, its performance deficiencies become evident when custom Python code is required to solve a problem.

Of course, there are many more programming languages, each with its own advantages and disadvantages, and some are optimized for specific use cases.

Nim is a state-of-the-art programming language well-suited for systems and application programming. Its clean Python-like syntax makes programming easy and enjoyable for beginners, without imposing any restrictions on experienced systems programmers. Nim combines successful concepts from mature languages like Python, Ada, and Modula with a few established features of the latest research. It offers high performance with type and memory safety while keeping the source code short and readable. Both the compiler and the generated executables support all major platforms, including Windows, Linux, BSD, and macOS. Cross-compiling to Android and other mobile and embedded devices and microcontrollers is possible, and the JavaScript backend allows the creation of web apps and to run programs in web browsers. The custom package managers, Nimble, Nimph and Atlas, facilitate the easy and secure use and redistribution of programs and libraries. The C, C++, and LLVM-based backends enable easy OS library calls without additional glue code, while the JavaScript backend generates high-quality code for web applications. The integration of the "Read/Eval/Print Loop" (REPL), "Hot code reloading", and incremental compilation (expected for versions > 2.0), along with support for various development environments — including debugging and language server protocols — make working with Nim both productive and enjoyable.

To maintain our motivation, let’s now present our first tiny Nim program. Ideally, we would delay this section until after installing the Nim compiler on our computer. However, we can already run and test the program by copying it into one of the available Nim online playgrounds like

There are two more unofficial sites that can run Nim code online:

In the section What is an algorithm? we described an algorithm to sum up the first 100 natural numbers. Converting that algorithm into a Nim program is straightforward, resulting in the text file provided below. You can copy it into the playground and run it now if you want. The program uses some basic Nim instructions, which we will briefly describe here. Everything will be explained in much more detail in the next part of this book.

We write Nim programs as plain text files using an editor tool, and you will learn how to create them soon. We call these text files the source code of the program. The source code is the input for the compiler. The compiler processes the source code, checks for obvious errors, and then generates an executable file that contains the final CPU instructions and can be run. Executable files are sometimes called executables or binary files. The term binary could be considered misleading, as all computer files are indeed stored as binary data. However, the expression 'binary' is used to differentiate executable programs from text files, such as Nim source code, which we can read, print, and edit using an editor. Don’t try to load the executable files generated by the Nim compiler into a text editor, as the content is not plain text, but numeric machine code that may confuse the editor. On the Windows OS, executable files typically get a special name extension .exe, but on Linux, no special name extensions are used.

Nim source code files are processed by the Nim compiler from top to bottom. In principle, for the generated executable, program execution also starts at the top. However, there are some exceptions to program execution; for example, program code enclosed in functions is not immediately executed where it appears in the source code file but rather when the function is called (invoked). And the program execution is not a linear process — we can use conditional expressions to skip parts of the program, or various loop constructs to repeat the execution of some program segments. In fact, the program execution in Nim is more similar to languages like Python or Ruby than to the C language: A C program always needs a main() function with exactly this name, and the execution of a C program always starts with a compiler-generated call to this function.

Variables are elementary entities of computer programs and are essentially named storage areas in the computer. As Nim is a compiled and statically-typed language, we have to declare each variable before we can use it. We do that by choosing a meaningful name for the variable and specifying its data type. To tell the compiler about our intention to declare a variable, we start the line with the var keyword, followed by the chosen name, a colon, and the data type of our variable. We have to put at least one space character between the var keyword and the name of the variable, to allow the compiler to recognize the two separate entities. Usually, we also put a space after the colon that separates the variable name from its data type. But this is only a convention to improve the readability of the source code. For the compiler, the colon already separates the variable name from the data type. The first line of our program declares a new variable named sum of data type int. int is short for integer and indicates that our variable should be able to store negative or positive integer numbers. (Integer numbers are whole numbers without a fraction, like -1, 0, 1234. Floating-point numbers, like 3.14159, represent another important numeric data type that we will use later as well.) The var at the start of the line is a keyword. Keywords are reserved symbols that have a special meaning for the compiler. Var indicates that we want to introduce a new variable. The compiler recognizes this and reserves a memory location in the computer’s RAM to store the actual value of the variable.

The second line is nearly identical to the first: we declare another variable, again of int type and a simple name, i.

Variable names like i, j, and k are typically used when we cannot think of a meaningful name or when we intend to use these variable as (array) indizes or as counters in loops. Note that in Nim, we can use arbitrary names for variables (with some restrictions) and that the actual name of a variable is not coupled to its data type or behavior. In early Fortran, that was handled differently, as the convention was that variables named i, j, and k were automatically of integer type by default.

In lines 3 and 4 of our program, we initialize the variables, that is, we give them a well-defined initial start value. To do that, we use the = operator to assign a value to the variable. Operators are special symbols like +, -, *, or / to indicate our desire to do an addition, a subtraction, a multiplication, or a division. Note that the = operator is used in Nim like in many other programming languages for assignment, and not like in traditional mathematics as an equality test. The reason for this is that, in computer programming, assignments occur more frequently than equality tests. Some early languages, like Pascal, used the compound := operator for assignment, which aligns more closely with mathematical usage. However, it is more difficult to type on a keyboard and is not visually appealing to most people. An expression like x = y assigns the content of variable y to x. In other words, x gets the value of y, the former value of x is overwritten and lost, and the content of y remains unchanged.

After such an assignment, x and y contain the same value. In the above example, we do not assign the content of a variable to the destination; instead, we use a literal numeric constant with the value 0. When the computer has executed lines 3 and 4, the variables sum and i each contain the start value 0. When we use the = operator for an assignment, we usually put a space character on both sides of the operator. However, this is merely a convention to improve the readability of the source code and is not strictly necessary. As a convention, spaces are typically placed on both sides of most Nim infix operators. This includes arithmetic operators, the assignment operator, and relational operators such as < or >. Also, similar to usage in ordinary text files, when we use a colon or a semicolon to separate two entities from each other, we usually put a space after the punctuation character.

Line 5 of our code example is much more interesting: it contains a while condition. The line starts with the term while, which is again a reserved keyword, followed by the logical expression i < 100 and a colon. An expression in Nim is something that produces a result, like the math expression 2 + 2, which yields the integer result of 4. A logical expression doesn’t yield a numerical result; instead, it yields a logical (boolean) result, which can be true or false. The logical expression i < 100 is dependent on the current value of the variable i. The two lines following the line with the while keyword are each indented by two spaces, meaning that these lines start with two additional spaces compared to the previous line. This form of indentation is used in Nim (and Python) to indicate blocks. Blocks are grouped statements. The complete while loop consists of the line containing the while keyword followed by a block of statements. The statement block after the while condition is executed as long as the while condition evaluates to the logical value true. For the first loop iteration i has the initial value 0, the condition i < 100 evaluates to the boolean value true, and the block after the while condition is executed for the first time. In this block, we have the inc() instruction. Inc is an abbreviation for increment. Inc(a, b) increases the value of variable a by b, while b remains unchanged. So in the above block, i is increased by one, followed by sum being increased by the current value of i. So when that block has been executed for the first time, i has the value 1 and sum also has the value 1. At the end of that block, execution starts again at the line with the while condition, now testing the expression i < 100 with i containing the value 1. Again, it evaluates to true, so the block is executed again; i then gets the new value 2, and sum becomes 3. This process continues until i reaches the value 100, at which point the condition i < 100 evaluates to false, and execution proceeds with the first instruction after the while block. That instruction is an echo statement, which is used in Nim to write values to the terminal or screen of the computer. Some other languages use terms like print or put instead of echo. You might still be wondering about the colon that terminates line five, which contains the while condition. That colon serves solely as a marker to indicate the end of a conditional statement.

Don’t worry if you haven’t understood much of this short explanation; we will explain all of it in much more detail later.

When we write numbers in everyday life, we typically use the decimal system with base 10, which includes the ten available digits 0 through 9. To get the value of a decimal number, we multiply each digit with powers of 10 depending on the position of the digit and sum the individual terms. The rightmost digit is multiplied by 10^0, the next digit by 10^1, and so on. A literal decimal number like 7382, therefore, has the numerical value 2 * 10^0 + 8 * 10^1 + 3 * 10^2 + 7 * 10^3. Here, we have used the exponential operator ^ — where 10^3 = 10 * 10 * 10. Current computers use binary representation internally for numbers. Generally, we do not care much about that fact, but it is good to know some properties of binary numbers. Binary numbers work nearly identically as decimal numbers. The difference is that we have only two available digits, which we write as 0 and 1. A number in binary representation is a sequence of these two digits. Like in the decimal system, the numerical value results from the individual digits and their position: The binary number 1011 has the numerical value 1 * 2^0 + 1 * 2^1 + 0 * 2^2 + 1 * 2^3, which is 11 in decimal notation. For binary numbers, the base is 2, so we multiply the binary digits by powers of two. Formally, the addition of two binary numbers works as we know it from the decimal system: we add the matching digits and take the carry into account, as in 1001 + 1101 = 10110, because we start by adding the two least significant digits of each number, which are both 1. That addition of 1+1 results in a carry and the resultant 0. The next two digits are both zero, but we have to take the carry from the former operation into account, so the result is 1. For the next position, we have to add 0 and 1, which is just 1 without a carry. And finally, we have 1 + 1, which results in 0 with a carry. The carry generates an additional digit, concluding the operation. In the decimal system with base 10, a multiplication with 10 is easily calculated by just shifting all digits by one place to the left and writing a 0 at the now empty rightmost position. For binary numbers, it is very similar: a multiplication by the base, which is two in the binary system, is simply a shift to the left, with the rightmost position filled by digit 0. ^[25]

In the binary system, the digits are typically called bits, and these bits are numbered from right to left, starting with 0 for the rightmost bit. For example, the binary number 10010101 is referred to as an 8-bit number because it requires eight digits to be represented in binary form. Often, individual bits are conceptualized as small bulbs, with a 1 bit represented as a lit bulb and a 0 bit represented as a dark bulb. A lit bulb is also referred to as a set bit. For instance, in the binary number 10010101, bits 0, 2, 4, and 7 are set, and the other bits are unset or cleared.

Groups of eight bits are called a byte, and sometimes, four bits are called a nibble. A word, which is an entity a computer can process in a single instruction, may consist of one, two, four, or eight bytes, depending on the CPU’s capacity. In the case of a CPU with an 8-byte word size, this means that the computer can, for instance, add two variables, each of 8-byte size, in a single instruction.

Let’s investigate some basic properties of binary numbers, starting with the assumption that we have an 8-bit word, also known as a byte. An 8-bit word can have 2^8 different states, as each bit can be set or unset independently of the other bits. That corresponds to the numbers 0 up to 255. For now, we’ll assume that we’re working with positive numbers only, but we will discuss negative numbers soon. An important property of binary numbers in computers is the wrapping around, which is a consequence of the fact that we have only a limited set of bits available to store the number. Therefore, when we continuously add 1 to a number, all bits eventually become set. This corresponds to the largest number that can be stored with that number of bits. When we then add again 1, we get an overflow. The run-time system may catch that overflow, so we might receive an overflow error, or the number is just reset to zero, as it may happen in our car when we manage to drive one million miles, or when the ordinary clock jumps from 23:59 to 00:00 of the next day. A useful property of binary numbers is the fact that we can easily invert all bits, that is, replace set bits with unset ones and vice versa. Let us use the prefix ! to indicate the operation of bit inversion, then !01001100 is 10110011. It is an obvious and useful fact that for each number x, we get a number with all bits set when we add x and !x. This means x + !x = 11111111 when considering an 8-bit word. Furthermore, if we ignore overflow, it follows that x + !x + 1 = 0 for each number x. This is a useful property that can be applied when considering negative numbers.

Now, let us investigate how we can encode negative numbers in binary form. In binary representation, only two states are available: 0 or 1, representing a set or an unset bit, respectively. But we have no unitary minus sign. The sign of a number could be encoded in the most significant bit of a word — if this bit is set, it indicates that the number is negative. Generally, a modified version of this encoding is used, called two’s complement: a negative number is constructed by first inverting all the bits — a 0 bit is transferred into a 1 bit and vice versa — and finally the number 1 is added. That encoding simplifies the CPU construction, as subtraction can be replaced by addition in this way:

Consider the case that we want to do a subtraction of two binary encoded numbers. The operation can be symbolically represented as A - B for arbitrary numbers A and B. Subtraction is, by definition, the inverse operation of addition. In other words, A + B - B = A, or B - B = 0 for every number B.

Assume we have a CPU that can do additions and that can invert all the bits of a number. Can we perform a subtraction with that CPU? Indeed, we can.

Remember that for each number X, X + !X + 1 = 0, provided we ignore overflow. If that relation is true for each number, then it is obviously true for each B in the expression A - B, and we can write A - B = A + (B + !B + 1) - B = A + (!B + 1), using the associative property of addition and subtraction in mathematics, that is we can group the terms as we want. But the term in the parenthesis is just the two’s complement, which we get when we invert all bits of B and add 1. So, to perform subtraction, we need to invert the bits of B and then add A, !B, and 1, ignoring overflow. That may sound complicated, but a bit inversion is a very cheap operation in a CPU, which is always available, and adding 1 is also a straightforward operation. The advantage is that we do not need separate hardware for the subtraction operation. Typically, subtraction in this way is not slower than addition because the bit inversion and the addition of 1 can be performed at the same time in the CPU as an ordinary addition.

From the equation above, indicating A - B = A + (!B + 1), it is obvious that we consider the two’s complement (!B + 1) as the negative of B. Note that the two’s complement of zero is again zero, and two’s complement of 00000001 is 11111111. All negative numbers in this system have a bit set to 1 at the leftmost position. This restricts all positive numbers to bit combinations where the leftmost bit is unset. For an 8-bit word, this means that positive numbers are restricted to the bits 00000000 to 01111111, which is the range 0 to 127 in decimal notation. The two’s complement of decimal 127 is 10000001. Seems to be fine so far, but note that there exists also the bit pattern 10000000, which is -128 in decimal. For that bit pattern, no positive value exists. If we try to construct the two’s complement of that bit pattern, we end up with the same pattern again. This is an asymmetry of two’s complement representation, which cannot be avoided. It generally is no problem, with one exception. We can never invert the sign of the smallest available integer, as that operation would result in a run-time error.^[26]

Summary: When working only with positive numbers, we can store numbers from 0 up to 255 in an 8-bit word, also known as a byte. In a 16-bit word, we could store values from 0 up to 2^16 - 1, which is 65535. When we need numbers that can also be negative, we have for 8-bit words the range from -128 to 127 available, which is -2^7 up to 2^7 - 1. For a signed 16-bit word, the range would be -2^15 up to 2^15 - 1.

While we can work with 8 or 16-bit words, for PC programming the CPU usually supports 32- or 64-bit words, so we have a much larger number range available. But when we program microcontrollers or embedded devices we may indeed have only 8- or 16-bit words available, or we may use such small word sizes intentionally on a PC to fit all of our data into a smaller memory area.

An important note to conclude this section is that whenever we have a word with a specific bit pattern stored in our computer’s memory, we cannot directly determine the type of data from the bit pattern. It can be a positive or a negative number, but maybe it is not a number at all but a letter or maybe something totally different. As an example, consider this 8-bit word: 10000001. It could be 129 if we have stored intentionally positive numbers in that storage location, or could be -127 if we intentionally stored a negative value. Or it could be not a number at all. Is that a problem? No, it is not as long as we use a programming language like Nim which uses static typing. Whenever we are using variables, we declare their type first, and so the compiler can do bookkeeping about the type of each variable stored in the computer memory. The benefit is that we can use all the available bits to encode our actual data, without having to reserve any bits to encode the actual data type of variables. For languages without static typing, this is not the case. In languages like Python or Ruby, we can use variables without a static type, so we can assign whatever we want to them. That seems to be comfortable at first but can be confusing when we write larger programs and the Python or Ruby interpreter has to do all the bookkeeping at runtime, which can slow down the program and consume additional memory.

Put another way, to determine if an operation is valid, it’s generally sufficient to know only the data type of the operands. We do not have to know the actual content. The only exception is if we invert the sign of the most negative integer number or if we perform an operation that causes an overflow, as there are not enough bits available to store the result — we may get a run-time error for that case.^[27] In a statically-typed language, each variable has a well-defined type, and the compiler can ensure at compile-time that all operations on that variable are valid. If an operation is not valid, the compiler will generate an error message. Then, when these operations are executed at run time, they are always valid operations, and the actual content, like the actual numeric value, does not matter (with the exception of overflow and perhaps a few other invalid math operations like division by zero).

Hexadecimal numbers, based on the 16-base numerical system, might seem less prevalent compared to binary numbers and their technical rationale may not be immediately apparent. However, these numbers continue to be relevant and you might come across them occasionally in various contexts. Originally, hexadecimal numbers emerged from the infancy of computer science when programming was primarily conducted through numerical codes rather than sophisticated programming languages. Despite their historical origin, hexadecimal numbers remain integral to modern computing. They serve as a more human-friendly representation of binary numbers, facilitating their comprehension and manipulation. This function has led to their extensive use in different areas of computing, including programming and networking. So, even though hexadecimal numbers are seen as a remnant from the nascent phase of computing, they retain their utility and relevance in contemporary computer science.^[28] To represent the 16 possible values of a hexadecimal digit, the 10 decimal digits 0 up to 9 are supplemented with the characters A through F. The most significant characteristic of a hexadecimal digit is that it can represent four bits — a unit equivalent to half of a byte, sometimes called a nibble. In the past, when manually entering binary numbers was necessary, it was often easier to encode a nibble using a hexadecimal digit:

The only place where we will encounter hexadecimal characters again in this book will be when we introduce character and string data types. There, control characters like a newline character are sometimes specified in hexadecimal form, such as "\x0A" for a newline character.

We will not go into great detail about installing the Nim compiler, as the process largely depends on your operating system, and the installation instructions may change in the future. We assume that you have a computer with an installed operating system and Internet access, and you are able to do at least very basic operations with your computer, such as switching it on, logging in, and opening a web browser or a terminal window. If that is not the case, then you should really seek help for these basic steps, and possibly with other basic tasks.

Detailed installation instructions are available on the Nim homepage at https://nim-lang.org/install.html.^[29] Try to follow these instructions. If they are not sufficient, please seek help in the Nim forum: https://forum.nim-lang.org/

If you are using a Linux operating system, then your system usually provides a package manager, which should make the installation very easy.

For example, on a Gentoo Linux system, you would open a root terminal and simply type emerge -av nim. That command would install Nim, including all necessary dependencies, for you. It may take a few minutes as Gentoo compiles all packages fresh from the source code, but then you are done. Similar commands exist for most other Linux distributions. This installation by a package manager installs Nim system-wide, so all users of the computer can now use Nim.

Another solution, which is preferable when you want to ensure that you get the most recent Nim compiler, is compiling directly from the latest git sources. This process is also straightforward and is described here: https://github.com/nim-lang/Nim. However, before you can follow these instructions, you must ensure that Git software and a working C compiler are installed on your computer.

Nim source code, like the source code of most other programming languages, is based on text files. Text files are documents saved on your computer that contain only ordinary letters, which you can type on your keyboard. This means no images or videos, no HTML content with fancy CSS styling. Generally, source code should contain only ordinary ASCII text, that is, no umlauts or Unicode characters.

To create source code, we typically use a text editor, which is a tool designed for creating and modifying plain text files. If you don’t already have a text editor, you could technically use a word processor to write your source code, though it’s not recommended. However, you would need to ensure that the file is saved as plain ASCII text. Editors typically support syntax highlighting, meaning keywords, numbers, and such are displayed with a unique color or style, making it easier to recognize the content. Some editors support advanced features like checking for errors while you type the program source code.

A list of recommended editors is available at https://nim-lang.org/faq.html

If you do not want to use a specialized editor now, then Gedit or Nano should be available on Linux. For Windows, you can use something like Notepad.

Typically, we store Nim source code files in their own directory, a separate section on your hard drive. If you’re working on Linux in a terminal window, you can type

You type these commands in the terminal window and press the return key after each of the above lines — that is, you type cd on your keyboard and then press the return key to execute that command. The same for the next three commands. What you have done is the following: you navigated to your default working area (home directory), created a subarea named mynimfiles, entered that subarea, and finally, launched the gedit editor. The argument test.nim tells gedit that you intend to create or modify a file called test.nim. If gedit is not available, or if you work on a computer without a graphical user interface, then you may replace the gedit command with nano. While gedit opens a new window with a graphical interface, nano opens only a very simple interface in the current terminal. Notable text editors without a GUI include Vim or NeoVim. These are very powerful editors, but they are difficult to learn, and they might seem unconventional as they have both a command mode and an ordinary text input mode available. For NeoVim, there is very good Nim support available.

If you prefer not to work from a terminal, or if you are using Windows or macOS, you should have a graphical user interface that allows you to create a directory and launch an editor.

Once the editor is open, you can type in the Nim source code from our previous example and save it as test.nim. Afterward, you can close the editor.

Note that the return key behaves differently in editors than in the terminal window: In the terminal window, you type in a command and finally press the return key to "launch" or execute the command. In an editor, the return key behaves similarly to the other keys: if you press ordinary keys in your editor, the corresponding character is added to your text, and the cursor moves one position to the right. And when you press the return key, then an invisible newline character is inserted, and the cursor moves to the start of the next line.

If you are working from a Linux terminal, then you can type

That is, you first list the content of your directory with the ls command, and then display the content of the Nim source code file that you have just typed in using the cat command.

Now type

This command invokes the Nim compiler and instructs it to compile your source code. The "c" letter is called an option or a sub-command. It tells the Nim compiler to compile your program and to use the C backend to generate an executable.

The compiler should display a success message almost immediately. If it displays error messages instead, you should relaunch Gedit or Nano, correct your typing error, save the modified file, and recompile.

When the source text is successfully compiled, you can run your program by typing

In your terminal window, a number will be displayed, which is the sum of the numbers 1 to 100.

If you haven’t been able to open a terminal to invoke the compiler, you might want to consider installing an advanced editor like VS-Code. These editors typically have the capability to launch the compiler and run the program directly within the editor.

The command

is the most basic compiler invocation. The extension .nim is optional, the compiler can infer that file extension. This command compiles our program in default debug mode; it uses the C compiler back end and generates a native executable. Debug mode means that the generated executable includes many checks, such as array index checks, range checks, nil dereference checks, among others. The generated executable may not run very fast, and it will be large, but if your program has bugs, then it will provide a meaningful error message in most cases. Only after you have carefully tested your program, you may consider compiling it without debug mode. You may do that with

The compiler option -d:release removes most checks and debugging code, and it enables the backend optimization by passing the option "-O3" to the C compiler backend, resulting in a very fast and small executable file. The option -d:danger includes -d:release and removes all checks. You should be aware that compiling with -d:danger means that your program may crash without any useful information, or even worse, it may run, but contain uncaught errors like overflows, which could lead to incorrect results. Generally, you should compile your program with plain nim c first. After you have tested it well, and if you need the additional performance, you may switch to the -d:release option. For games, benchmarks, or other non-critical tasks, you may try the option -d:danger, to get an executable without any checks for utmost performance.

There are many more compiler options. You can find explanations for them in the Nim language manual, or you can display them using the commands nim --help and nim --fullhelp. An important new option is --mm:arc, which enables the new deterministic memory management. You could combine --mm:arc with -d:useMalloc to disable Nim’s own memory allocator. This reduces the executable size and enables the use of Valgrind to detect memory leaks. Similar to --mm:arc is the option --mm:orc, which can additionally deal with cyclic data structures. Another powerful option is --passC:-flto. This option is for the C compiler backend and enables link time optimization (LTO). LTO enables inlining for all procedure calls and can significantly reduce the final program size. In recent versions of the Nim compiler, -d:lto can be used instead of --passC:-flto. Furthermore, for Nim v2.0, --mm:orc is the default memory management strategy. It’s worth mentioning that you can also try the C++ compiler backend using the cpp sub-command instead of the plain c command. Additionally, you may compile with the CLang backend instead of the default GCC backend using the --cc:clang option. You can additionally specify the option -r to immediately run the program after a successful build. For testing small scripts, the compiler invocation in the form nim r myfile.nim can be used to compile and run a program without generating a permanent executable file. Here’s an example of how you can use all these options:

In this example, -march=native is additionally passed to the C compiler backend to enable the use of the most efficient CPU instructions of your computer. This could result in an executable that won’t run on older hardware. Of course, you can save all these parameters in configuration files, eliminating the need to type them in for each compiler invocation. You may find more explanations for all the compiler options in the Nim manual, or in later sections of this book; this includes the options for the JavaScript backend.

Before concluding this introduction, we should mention that the Nim language supports stropping for keywords and operators by enclosing them in backticks. This way, it is possible to use Nim keywords like type, from, or object as ordinary symbols, for example, as variable or field names. Typically, we avoid using keywords as ordinary symbols. However, when interfacing with C libraries, there may be instances where these libraries use symbols that are keywords in Nim. So, instead of renaming the symbols, we could use a notation like `object` for a proc parameter name, or `from` for a field name. Actually, we have to use stropping when we define procedures and functions that serve as operators, as in the example proc `*`(c: char; i: int): string = c.repeat(i).

In the Nim programming language, declarations serve a significant role by allowing us to define constants, variables, procedures, and even our unique data types. Declarations serve to inform both the compiler and the human reader about crucial attributes such as the name and data type of the variable we intend to use. Being a statically and strongly typed language, Nim requires this information for the compiler to function correctly. These declarations are not only useful to the compiler but also prove beneficial for us as programmers. They act as compact references, simplifying the process of understanding and managing the code. This is particularly valuable when collaborating with others, as it ensures clear communication and consistency in coding style, fostering a more effective development environment.^[30]

We will explain the type and procedure declarations in later sections. For now, we will focus on constant and variable declarations.

A constant declaration in its simplest form maps a symbolic name to a value, like

We use the reserved keyword const to inform the compiler that we want to declare a constant named Pi and assign it the numeric decimal value 3.14159. Nim has a small set of reserved keywords such as var, const, proc, and while, among others, which tell the compiler that we want to declare a variable, a constant, a procedure, or that we want to use a while loop for some repeated code execution. Reserved keywords in Nim are specific symbols that hold special significance for the compiler. Therefore, we should avoid using these symbols as names for other entities such as variables, constants, or functions to prevent confusion for the compiler. The symbol = is the assignment operator in Nim; it assigns the value or expression on its right side to the symbol on its left. You have to understand that this assignment operator is different from the equal sign we may use in mathematics to express an equality relation. Some languages, like Pascal, initially used the compound operator := for assignments. However, this can be challenging to type and may confuse individuals unfamiliar with it. Since source code typically contains many assignments, using the symbol = is quite sensible. For the actual equality test of two entities, which is not used that often, we use the compound == operator in Nim, as in most other programming languages including C and Python. We call = an operator. Operators are symbols that execute basic operations, like + for addition of two numbers, or = for assignment of a value to a symbol. Most operators are used as infix operators between two arguments, as in the expression 2 * Pi, which denotes the multiplication of the named constant Pi with the literal number 2, resulting in the floating-point value 6.28318. However, operators can also function as unary operators, such as in -Pi where in unary minus inverts the sign of a numeric value. When declaring named constants, we must always assign a value immediately. That value can never change, but of course, we can use the named constant in expressions to derive different values, as in

When declaring constants, you can also specify the exact data type of the constant value, as in

Typically, specifying the type isn’t necessary, as Nim employs type inference. From the literal value 3.14, it is obvious that it is a decimal floating point number. For the second line, type inference would conclude that the constant Two is of integer type, as no fractional part is given. In this case, we can specify the desired data type after the name of the constant, separated by a colon. Alternatively, we could write const Two = 2.0. When dealing with numeric expressions with constants, the Nim compiler performs intelligent automatic type promotion. For instance, when given the expression const TwoPi = 2 * Pi, Nim assumes that what we actually intended was const TwoPi = 2.0 * Pi.

For numeric expressions with variables, this type-promotion is stricter. It aims to avoid unnecessary type conversions at runtime and to ensure that the final program truly utilizes the intended data types.

As mentioned in Part I of the book, we usually place a space on either side of an operator when we use it in infix notation between two operands. This convention improves the readability of the source code. As mentioned before, in Nim, spaces can sometimes change the interpretation of an expression. This is because Nim adheres to the conventions of handwritten notation. For instance, a + -b is significantly different from a+-b. We will discuss these notations in later sections of the book in more detail.

With the aforementioned constant declaration, we can use the symbol Pi in our program’s source code, eliminating the need to remember or retype the exact sequence of digits. Utilizing named constants, such as Pi from our previous example, simplifies value modification. If we need more precision, we can update the exact value of Pi in one place in our source code, rather than searching for the digit sequence 3.14 throughout our code files.

For numeric constants, such as our Pi value, the compiler will substitute the symbol with its actual numeric value in the source code during compilation.

Expressions assigned to constants are already evaluated at compile time. Thus, complicated constant expressions do not negatively impact the program’s performance. The expressions can contain simple operations like basic math, and most Nim functions can be used as well, but functions like sin() from external C libraries might currently be unavailable.

Variable declarations are more complex because they require the compiler to reserve a specific named storage location:

In this case, we place the reserved keyword var at the start of the line to indicate to the compiler that we are declaring a variable. We then give the variable our chosen name, followed by a colon and the data type. The int type is a predefined numeric data type indicating a signed integer. The storage capacity of an integer variable depends on the operating system of your computer. On 32-bit systems, 32 bits are used, and on 64-bit systems, 64 bits are used to store one single integer variable. This range is adequate even for large signed integers, with a range from -2^31 to 2^31 - 1 for 32-bit systems, and from -2^63 to 2^63 - 1 for 64-bit systems.

While we generally use lower-case names for variables, the names of constants can start with an uppercase letter as well.

Variables declared using the var keyword act as simple containers, storing a value which can be accessed or modified later. We can assign an initial value to the variable immediately when we declare it, similar to how we do it for constants, or we can assign the value later. If no actual value is assigned to the variable, it assumes a default value, which for numeric variables is zero:

In the first and second lines, we declare two variables, start and stop, both of which initially hold the default integer value of zero. In the third line, we declare one more integer variable called delta, to which we assign an initial value of 3. And finally, in the fourth line, we assign an integer expression to the variable stop. Nim offers more variants for variable declarations, which we will discuss shortly. These include utilizing type inference when immediately assigning an initial value, using var sections to declare multiple variables without repeating the var keyword, listing multiple names of the same data type in front of the colon separated by commas, or using the let keyword to declare immutable variables.

Nim v2.0 introduces the strictDefs pragma, which can enforce variable initialization. This helps avoid errors that might occur when variables default to zero but require a different initial value. The strictDefs pragma, along with other new features of Nim 2.0, is described in detail in the book’s Appendix.

Statements, or instructions, are a core component of Nim programs; they tell the computer what it shall do. Often statements are procedure calls, like the call of the echo() or inc() procedure, which we have already seen in Part I of the book. We will learn what procedures exactly are in later sections. For now, we can consider procedures as entities that perform specific tasks when we call (or invoke) them. We invoke them by writing their name in our source code file, followed by a list of parameters, or arguments. When we write echo 7, echo() is the procedure that we call, and 7 is the argument — an integer literal in this case. When the parameter list includes more than one argument, we separate the arguments with a comma and typically add an optional space afterward. As a result of our procedure call, the decimal number 7 is written to the terminal window when we execute the compiled program. The parameter list can be empty, and the parameters can be expressions, that may again contain function calls like echo sin(0) + 2.0. In contrast to languages like C, where the parameter list must always be enclosed in brackets, Nim often allows us to omit the brackets — a feature known as the command invocation syntax.

The command invocation syntax is typically used with the echo() procedure, or when a procedure has only a single argument. For multiple arguments, or when the argument is a complicated expression, the use of brackets is preferable. In some programming languages, like C, coding styles may suggest placing a space between the function name and the opening bracket. For Nim, we should not do that, the reason will become clear when we later explain the tuple data type. A few procedures have no parameters at all. When we call these procedures, we always have to use the syntax myProc() with an empty pair of brackets to make it clear to the compiler that we want to call that procedure. The statement res = myProc() assigns the result of the procedure call to res, while res = myProc assigns the procedure itself to res, which is a significantly different operation.

Functions are a special form of procedures that return a value or a result. For instance, in mathematics, sin() or cos() are functions — we pass an angle as an argument and they return the sine or cosine of that angle, respectively. On the other hand, the echo() procedure, which prints the arguments, is not a function as it doesn’t return a result.

Let’s examine this minimal Nim program:

The Nim program above consists of a variable declaration and three statements: in the first line, we declare the variable we want to use. In the next line, we assign the value 2 + 3 to it, and finally, in line 3 we use the procedure echo() to display the content of our variable in the terminal window. In the last line, we once again use the echo() procedure with a conventional parameter list enclosed in brackets. The parameter list contains a single argument, which is the sum of a function call to cos(0) and the literal value 2. Here, the compiler would first call cos(0), then add the literal value 2 to that result, and finally pass the sum to the echo procedure to print the value. ^[31]

Nim programs are generally processed from top to bottom by the compiler, and they also execute in the same order after successful compilation. A consequence of this is that we have to write the lines of the above program exactly in that order. If we moved the variable declaration down, then the compiler would complain about an undeclared variable because the variable is used before it has been declared. If we exchanged lines 2 and 3, then the compiler would be still satisfied, and we would be able to compile and run the program. However, the output would be significantly different because the uninitialized value of the variable a would be displayed first and only then would it be assigned a value.

When we have to declare multiple constants or variables, we can use a block. That is, we write the keyword var or const on its own line, followed by the respective declarations as shown:

These blocks are also referred to as sections, for example, const section or var section, as is customary in Wirthian languages. Take note of the indentation — the lines following const and var begin with a few spaces, forming an indented block that allows the compiler to identify the end of the declaration. Typically, we use two spaces for each level of indentation. While other numbers of spaces can be used, it’s essential to maintain consistency in the indentation scheme. Two spaces are generally recommended as they are easily recognizable in the source code and do not consume excessive space; thus, they do not create overly lengthy lines that may not fit on the screen.

Also note that in Nim, we generally write each statement on its own line. The line break indicates to the compiler that the statement has ended. Special statement delimiters as the ; in C are not require at the line end, but can be used to separate multiple statements on the same line. There are a few exceptions to this rule — for example, long mathematical expressions can continue on the next line. Generally, when a line ends with a punctuation character, and the next line is indented, the compiler recognizes the continuation. (for more details, refer to the Nim manual). Multiple statements can also be put on a single line by separating them with a semicolon:

It is also possible to declare multiple variables of the same type in a single declaration, as shown below:

Alternatively, we can assign an initial start value to a variable as shown in the example below:

Nim also currently supports the initialization of multiple variables with the same value:

Here, both i and j would get the initial value 1. However, this notation is often avoided as it may not be immediately clear to all readers.

Lastly, we can use type inference for variable declarations when an initial value is assigned, as shown in the example below:

The compiler recognizes in this case that we assign an integer literal to that variable, and so silently gives the variable the int type for us. Type inference can be convenient, but it might make the source code more difficult for readers to understand, or the type inference might not always yield the expected results. For example, in the above code, year gets the data type int, which is a signed 4 or 8-byte number. However, we might prefer an unsigned number or a number that occupies only two bytes in memory. For the final executable, it makes no difference whether a variable received its runtime type through direct user specification or by the use of type inference, as long as the actual data type is the same. Although the use of type inference may slightly increase the compile time for our source code, this increase is typically negligible.

Note: For integral data, we mostly use the int data type in Nim, which is a signed type with a 4 or 8-byte size. It usually does not make sense to use many different integral types — signed, unsigned, and types of different byte sizes. Mixing them in numerical expressions can be confusing and potentially even decrease performance, because the computer may have to do type conversion before it can do the math operation. Another problem associated with unsigned types is that mathematical operations on unsigned operands could yield a negative result. Consider the following example, where we use a hypothetical data type "unsigned int" to indicate unsigned integers:

The true result should be -4, however, a is an unsigned type and cannot contain a negative value. So, what should happen — an incorrect result or a program termination?

Another aspect related to variable declarations is the initial value of variables. Upon declaration, Nim resets all the bits of our variables. This means that numerical variables automatically have an initial value of zero unless we assign a different value in the variable declaration.

In this declaration

both variables get the initial value of zero.

We have already mentioned that Nim 2.0 introduces the strictDefs pragma, which enforces explicit initialization. That is explained in more detail in the Appendix where we summarize all the new 2.0 features.

There is a variant for variable declarations that uses the let keyword instead of the var keyword. Let is used when we need a variable that gets assigned a value only once, while var is used when we anticipate changing the content of the variable during the program execution. We say that we use var to create mutable variables, and let to create immutable variables. Let seems to be similar to const, but in const declarations, we can only use values that are known at compile time. Let permits us to assign values to variables that become available only at program runtime, possibly because the value derives from a previous calculation. However, let also indicates that the assignment occurs only once, and the content does not change later during the program’s execution. We refer to such a variable as immutable. The use of the let keyword can aid in understanding the source code and potentially help the compiler optimize for faster or more compact code. For now, we can just ignore let declarations and use var instead — later, we may use let where appropriate, and the compiler will tell us when let will not work, and we have to use var.

With the knowledge we have gained in this section, we can rewrite our initial Nim example from Part I as follows:

In the code above, we declare both int type variables in a single line and take advantage of the compiler initializing them to 0. We also use a named constant for the upper loop boundary. Another tiny fix is that we write inc(i) instead of inc(i, 1). We can do that because there exist multiple procedures with the name inc() — one which takes two arguments, and one which takes only one argument and always increases that argument by one. Procedures with the same name but different parameter lists are referred to as overloaded procedures. Instead of inc(i), we could have written also i = i + 1, and instead of inc(sum, i) we could write sum = sum + i. Either form would generate identical code in the executable, so it’s a matter of personal preference.

We have already used the echo() procedure for displaying textual output in the terminal window. In previous code examples, we passed integer type arguments to the echo() proc. This procedure automatically converted these integers into a textual sequence of decimal digits for display in the terminal window. In the Nim programming language, text is represented by a predefined, built-in data type known as a string. We will delve into the details of the string data type in the next section. For now, it’s sufficient to know that it exists and we can use the echo() proc to print text strings. The echo() procedure is capable of automatically converting other data types, such as numbers or Boolean values (true/false), into human-readable text strings for terminal output. Recall that most data types are stored internally in our computer as bits and bytes, which have no true human-readable representation by default. Numbers, like most other data types stored in the computer, are essentially abstract entities. As we’ve learned, all data in a computer is stored internally in binary form, which means it’s stored as a bit pattern of 0s and 1s. However, even that bit pattern is an abstraction. We would require a procedure that prints a 0 for each unset bit and a 1 for each set bit to display the content of an internally stored number in binary form in the terminal or elsewhere. Similarly, we require a procedure to print an internally stored number as a human-readable sequence of decimal digits. Even text strings are stored internally as abstract bit patterns and require conversion procedures to be rendered as readable text. The echo() procedure is capable of accomplishing all this, although we will not delve into these details at this point.

For our subsequent experiments, we may want to input some user data in the terminal. As we do not know much about the various available data types and the procedures that can be used to read them in, we will just present a procedure that can read a text string that the user types in the terminal window. We will utilize the readLine() function for this task.

Please note that the return key must be pressed after entering your text.

The first line of our program demonstrates how we can print a literal text string with the echo() proc. To mark text literals unambiguously and to separate them from other literals like numeric literals or from variables, the string literals have to be enclosed in quotation marks. In the second line of our example program, we use the readLine() function to read textual user input. Note that we call readLine() a function, not a procedure, to emphasize that it returns a value. The readLine() function requires one parameter to specify the source of the input — for instance, the terminal window or a file. The stdin parameter directs the function to read from the current terminal window. Notably, stdin is a global variable of the system (io) module and represents the standard input stream. Finally, in line 3 we use again the echo() proc to print some text. In this case, we pass two arguments to echo(): a literal text enclosed in quotes, and the mytext variable, separated by a comma. The mytext variable has the data type string. In this example, we employed type inference to declare the data type. Since the readLine() function always returns a string, which is known to the compiler, our mytext variable is automatically declared as a string. We will learn more about the data type string and other useful predefined data types in the next section.

When you try the example code from above, you might want a variant that reads the textual input not on its own line but directly after the prompt, such as 'What is your name: Nimrod'. As the echo() proc always writes a newline character after the last argument has been written, we have to use a different function to get the input prompt on the same line. We can use the write() proc from the system module for this. As write() can not only write to the terminal but also to files, it needs an additional parameter that specifies the destination. We can pass the variable stdout from the system module to indicate that write() should write to our terminal window. Often, beginners also desire the ability to read single-character input without the additional need to press the return key. For that, we can use the getch() function from the terminal module — that function waits (blocks) until a key is pressed and returns the ASCII character of the pressed key:

Don’t be misled by the fact that the first write() call and the subsequent readline() call do not appear on the same line in our example. In this case, the actual format of our source code does not influence the program output. We could write both function calls on a single line, separated by a semicolon. But that would make no difference for the program output. The key difference between the two function calls above is that write() prints the text without advancing the cursor to the next line in the terminal window, while echo() does so once all arguments have been printed. We say that echo() prints automatically a '\n' character, which we call a newline character, after all the arguments have been printed.

Nim is a statically typed programming language, which means that all variables have a well-defined data type, and this data type does not change during program execution. Moreover, we say that Nim is a strongly typed language, meaning that it does nearly no automatic type conversions when variables are assigned to each other or used in expressions or as arguments in function calls. Automatic type conversion may seem beneficial at first, but it can easily introduce errors or degrade the performance of our programs.

The most fundamental data type — in real life and in computer science — is the integer (whole) number. All other numeric data types, like fractional, floating-point, or complex numbers, and other fundamental types like the boolean type with its two values true and false, and character and text string types, can be represented as integers. For that reason, both the early computers built in the 1950s and today’s smallest microcontrollers work internally only with integer numbers. The integer data type is not only crucial for arithmetic operations, but it is also used as an index to access elements in data structures such as arrays. Furthermore, integer numbers are often interpreted as bit vectors to represent set-like data types. As all CPUs are able to do basic bit operations like setting or clearing individual bits, and as bit patterns map well to mathematical sets, set data types are well-supported by all CPUs, and so set operations are generally very efficient. Advanced computers, built in the 1980s, received support for the crucial class of floating-point numbers through specialized floating-point processors for fast numerical computations. Today, these floating-point units are typically integrated into the CPU, and GPUs can even process many floating-point operations in parallel. However, the precision of GPUs is typically limited to the ranges needed for games and graphic animations; that is, 32- or even 16-bit. Modern CPUs often also have some form of support for vector data types to process multiple values in one instruction (SIMD, single instruction, multiple data).

Non-numeric types like characters or text strings are internally represented by integer numbers. In the C language, the data type to represent text strings is called char, but it is indeed only an 8-bit integer type that supports all the mathematical operations defined for ordinary integer types. In Nim and the Wirthian languages, most math operations are not directly allowed for the char data type, which helps prevent misuse and allows the compiler to catch logical errors.

Nim also supports several built-in homogeneous container types like arrays and sequences, along with numerous built-in derived types like enumeration types, sub-ranges and slices, distinct types, and view types (experimental). The built-in inhomogeneous container types object and tuple, which allow grouping of other types, are complemented by a variant type container, which allows instances of that type to contain different child types at runtime. These inhomogeneous container types are similar to the struct and union types from the C programming language.

Other basic and advanced data types like complex and fractional numbers, types with arbitrary-precision arithmetic, as well as hash sets and hash tables, dynamically linked lists, or tree structures are available through the Nim standard library or external packages. Of course, we are also able to define our own custom data types with our own operators, functions, and procedures working on them.

Note that all the data types that are built into the language, like the primitive types int, float, or char, as well as the built-in container types like tuple, object, seq, and string, are written in lower case, while data types that are defined by the Nim standard library or that we define ourselves, by convention, start with a capital letter like the CountTables type defined in the tables module. Some people may regard this as an inconsistency, while others may say that this distinction allows us to differentiate built-in types from types defined by libraries.

At least, we can agree that using capital notation for common types such as Int, Float, or String would be more difficult to type and wouldn’t look as nice.

You have already seen a few examples of simple Nim source code. The code essentially consists of a plain text file made up of ASCII characters - that is, the ordinary characters that you can type on your keyboard. Generally, Nim source code can also contain Unicode utf-8 characters, so instead of using names consisting of ASCII characters for your symbols, you could just use single Unicode characters or sequences of Unicode characters. However, this typically doesn’t make much sense as entering Unicode isn’t easy with a keyboard. Additionally, it can only be displayed correctly on the screen or in the terminal if the editor or terminal properly supports Unicode and all necessary fonts are installed. This may be possible on your local computer, but what happens when someone else edits your source code?

Starting with Nim version 1.6, we received support for Unicode operators, which could be useful for some applications. For details, please see the Nim language manual.

Nim currently does not permit the insertion of tabular characters (tabs) in your source code, so you must indent blocks using spaces only. Typically, we use two spaces for each indentation level. Other quantities also work, but it’s best to stick to a consistent number.

Identifiers in Nim, as used for modules, variables, constants, procedures, user-defined types, and other symbols, can contain lowercase and uppercase letters, digits, Unicode characters, and additional underscores. However, names must not start with digits or begin or end with an underscore, and one underscore may not immediately follow another underscore.

Generally, we use camel-case like leftMargin for variable names, not snake-case like left_margin.

Current Nim has the special property that identifiers are case-insensitive and that underscores are simply ignored by the compiler. The only exception is the first letter of a name; that letter is case-sensitive. So the names leftMargin, leftmargin, and left_margin are identical for the compiler. But LeftMargin is different from all the others because it starts with a capital letter. This may sound a bit strange at first but works well in practice. One advantage is that a library author can use snake_case in their library for names, while users of the library can freely decide if they prefer camelCase. Still, you might think this could generate confusion. In practice, it does not, it prevents confusion. Imagine a conventional programming language that is fully case-sensitive and does not ignore underscores. In a larger program, we could then have names like nextIteration and next_Iteration or keymap and keyMap. What when both names are visible in the current scope, and we type the wrong one? The compiler may not detect it when types match, but the program may do strange things. Nim would not allow such similar-looking names, as the compiler would regard them as identical and would complain about a symbol redefinition.

You might wonder why the first letter is case-sensitive. This is to allow user-defined types to use capital letters in their names and then write something like var window: Window. So we can declare a variable named window of a user-defined data type named Window. That is a common practice.

The case insensitivity and the ignoring of underscores may not be the greatest invention of Nim, but it does not really hurt. The only exception occurs when we create bindings to C libraries where leading or trailing underscores are used, necessitating some renaming.

The only minor disadvantage of Nim’s fuzzy names arises when using tools like Grep or your editor’s search functionality. You cannot be certain if a search for "KdTree" will yield all results; you might have to try "Kd_Tree" or "KDTree" and potentially some other variants as well. To address this issue, Nim provides a tool called nimgrep that conducts a case- and style-insensitive search. Your editor may also support that type of search. You can enforce a consistent naming scheme by calling the compiler with the command-line argument --styleCheck:error or --styleCheck:hint.

Larger computer programs generally consist not only of code that is executed linearly but also of code for conditional or repeated execution.

The most important control structures of Nim are the if statement for conditional execution, the related case statement, and the while and for loops for repetitions. All these statements control program execution at runtime. Nim’s when statement, which is syntactically very similar to the if statement, is evaluated at compile-time. It can be used to adapt our program code for various operating systems or to compile our code with special options, such as for debugging or testing purposes.

All these control structures can be nested in arbitrary ways, so we can have in one if branch other if conditions or while loops, and in while loops again other control structures including other loops.

We have worked with basic data types like numbers, characters, and strings already. Often it makes sense to join some variables of these basic data types to more complex entities. Assume you want to build an online store to sell computers and build a database for them. The database should contain the most important data of each device type, like the type of CPU, RAM and SSD size, power consumption, manufacturer, quantity available, and actual selling price.

We can create a custom object data type with fields containing the desired data for this purpose:

In the first line, we use the type keyword to tell the compiler that we want to define a new custom type. Writing the type keyword on its own line begins a type section where we can declare one or more custom data types. All type declarations in a type section must be indented. In the next line, we write our type name, an equal sign, and the keyword object. This indicates that we want to declare a new object type named Computer. Here, Computer is a type name; in Nim, we use the convention that user-defined type names start with a capital letter. In the following indented block we specify the desired fields of this object, each line contains the name of a field and a colon followed by the needed data type. That is similar to a plain variable declaration.

Objects in Nim are similar to structs in C. Unlike classes in Java, Nim objects contain only the fields, sometimes also called member variables, but no procedures, functions, or methods, and no initializers or destructors as in C++. In Nim, we keep the data objects separate from the procedures, functions, methods, and also optional initializers and destructors that work with those data objects.

Now that we have defined our own new object type, we can declare variables of that type and store content in its fields.

Of course, in real applications, we would fill the fields not in this way, but we would maybe read the data from a file, from a terminal, or maybe from a graphical user interface.

It may look a bit ugly that we have to write computer. before each field when we access the fields. Indeed, in recent Nim versions, this is not necessary; you may use the with construct instead.

We can use the fields like ordinary variables:

As mentioned earlier, the right side of the assignment operator is evaluated first, then the result is stored in the variable on the left side. But we can also just write computer.quantity -= 1 or dec(computer.quantity).

Objects, like all other data types that we have already used, are value types, which means that when an object is assigned to a new variable, all its components are copied as well. In this way, objects behave like strings — assignment copies the content, with the entities remaining independent of each other. We will learn about reference types soon, which behave differently.

To initialize object variables, we can use the object type names as a constructor with a syntax like Foo(field: value, …​). Unspecified fields get the field type’s default values:

To initialize the variable computer1, we used the constructor syntax. In line five, we use the assignment operator to copy the content of variable computer1 into variable comp2, and finally, we overwrite the price field in comp2. As both variables are distinct instances, the fields of variable computer1 are not modified this way.

Starting with Nim v2.0, object fields can have custom default values, instead of the binary zero. The syntax for the defaults is the same as the assignment for ordinary variables, as shown below:

Note that custom default values for object fields are only applied, when we explicitly initialize a variable by use of an object constructor, or when we use the default() function for the initialization. A plain variable declaration like var c2: Computer initializes all fields to binary zero". This behaviour could be surprising, and perhaps it would be a good idea when the compiler gives a warning when for objects with default field values a plain declaration like var c2: Computer is used.

Typically, a computer store would offer many different types of computers, so it would make sense to store all the different devices in a container like a sequence, called short seq in Nim. In the next section, we will learn how we can do that.

Sequences and arrays are homogeneous containers. They can contain multiple elements of the same data type, while a plain variable, such as a float or an int, only contains a single value. In some ways, we can regard objects as containers as well because objects contain multiple fields. The same holds for tuples — tuples are a very simple, restricted form of objects and also contain fields. But more typical container data types are the built-in arrays and sequences, or for example, hash tables, which are provided by the Nim standard library. Arrays, sequences, and hash tables can contain multiple elements, but all elements must have the same data type, which we call the base type.^[41] The data type of the base type is not restricted; it can even be an array or sequence type again, allowing us to build multidimensional matrices in this way. Arrays have a fixed, predefined size; they cannot grow or shrink during the runtime of our program. Sequences and hash tables can grow and shrink.

Arrays and sequences appear very similar. A sequence seems even more powerful because it can change its size, i.e., the number of elements it contains, at runtime, while an array has a fixed size. So why do we have arrays at all? The reason is mostly efficiency and performance. An array is a plain block of memory in the RAM of the computer, which can be accessed very fast and needs not much care by the runtime system. Sequences require much more effort, especially when we add elements and the sequence needs to grow. When we create sequences, we can specify how many elements should fit in it at least, and the runtime system reserves a block of RAM of the appropriate size. But when our estimation was too small, and we want to append or insert even more elements, then the runtime system may have to allocate a larger block of memory first, copy the already existing elements to the new location, and then release the old, now unnecessary memory block. And this is a relatively slow operation. The reason this process may be necessary is that the initially allocated memory block may not be able to increase in size if the neighboring space in the RAM is already occupied by other data. Now, let us see what we can do with arrays and sequences:

In the second line of the code above, we declare a variable named a of array type — we want to use an array with exactly 8 elements, and each element should have the data type int. To declare a variable of array data type we use the array keyword followed in square brackets by the number of the elements, and separated by a comma, the data type of the elements. We can also specify the range of the indices explicitly by specifying a range like array[0 .. 7, int] or array[-4 .. 3, int]. The first specification is identical to the one in the above example program, and the second one would allow us to access array elements with index positions from -4 up to 3.^[42]

When we declare an array instance variable, then all the contained elements get the default value binary zero. But we can also explicitly assign initial values like a: array[8, int] = [1, 2, 3, 4, 5, 6, 7, 8]. Here the expression on the right is Nim’s array constructor. Whenever we use an array constructor to initialize an array instance variable, then the number of elements that the constructor provides has to match the size of the array variable, and the element types have to match as well. To specify the element type of an array constructor, it is often enough to specify the type of the first element, so [1.int8, 2] is equivalent to [1.int8, 2.int8]. We can use for loops to iterate over all the elements of an array, in a similar way as we did it for strings. The first for loop of the above program fills our array — that is, for each of the 8 storage places in the array, we fill in some well-defined data. We use the mitems() iterator here because we want to modify the content of our array — we fill in numbers 1 .. 8. In the next for loop, we square each storage location, and finally, we print the content. In the last for loop, we do not modify the content, so a plain items() instead of mitems() would work, but we have already learned that we don’t need to write the plain items() at all in this case.

Sequences, called just seq in Nim, work very similarly to arrays, but they can grow:

We start with an empty seq here and use the add() proc to append elements. After that, we can iterate over the seq as we did for the array.

In the same way as we access single characters of a string with the subscript operator [], we can use that operator to access single elements of an array or a seq, as in a[myPos]. The slice operator is available for arrays and sequences too and can be used to extract sub-ranges or to replace multiple elements. Because arrays have a fixed length, the slice operator can only replace elements in them, but not remove or insert ranges. The first element position is generally 0 for arrays and sequences. Arrays can even be defined in a way that the index position starts with an arbitrary value, but that is not used that often. Whenever you use the subscript or slice operator, you have to ensure that you access only valid positions, that is, positions that really exist. a[8] or s[8] would be invalid in our above example — the array has only places numbered 0 .. 7, and for the seq, we have added 8 values which now occupy positions 0 .. 7 also, position 8 in the seq is still undefined. We would get a runtime error if we tried to access position 8 or above, as well as if we tried to access negative positions. You might think that an assignment for a seq, such as s[s.length] = 9, is the same as s.add(9), but only the add() operation works in this case.

Note that in some languages like Julia arrays start at position 1.^[43] Nim arrays can have an arbitrary integral start position, including negative start positions, but the start position as well as the highest subscript position are determined in the program source code and can not change at runtime. We say that arrays have fixed compile-time bounds. Sequences always start at position 0, we can specify an initial size, and we can always add more elements at runtime.

Arrays and sequences allow fast access to their elements: All the elements are stored in a contiguous memory block in RAM, and the start location of that memory block is well-known. As all the elements have the same byte size, it is an easy operation to find the memory location of each element. The compiler uses the start location of the array or seq, and adds the product of subscript index and element byte size. The result is the memory location of the desired element, which was selected by the index used in the subscript operator. When the array should not start at position 0, then the compiler would have to adjust the index, by subtraction of the well-known start index. This operation doesn’t take much time, but nonetheless, arrays starting at position 0 can be slightly faster. As mentioned earlier, the compiler must perform a multiplication operation between the index position and element size — a task that involves integer multiplication and is consequently quite fast. When the element size is a power of two, then the compiler can even optimize the multiplication by using a simple shift operation, which can be even faster, depending on the CPU being used.

It should not be surprising that the internal structure of sequences is a bit more involved than that of arrays. Arrays are indeed nothing more than a block of memory, generally allocated on the stack for local data or allocated in the BSS segment for global data. Don’t worry if you do not yet have an idea of what the stack, the heap, and a BSS segment are; we will learn about them soon. The Nim seq data type, having a variable size, clearly requires not just a storage location for its elements, but also a counter to track its current number of elements and another counter for its maximum capacity. The element counter must be updated when we add or delete elements, and when the counter tells that there is currently no more space available for more elements, then a new block of memory must be allocated, and the existing elements must be copied from the old location into the newly allocated memory region before the old memory region can be released.^[44] Due to this additional effort appending elements to a seq by using the add() proc is not extremely fast. You may wonder why we do not have to save size information for arrays. Well arrays have fixed sizes, so it is obvious that we never have to adjust something like a size counter, simply because the size would never change. But should we store the desired initial size of the array? In a way, yes. However, it is a constant value. During the compilation process, the compiler can already catch some errors for us — if we have an array as above with size 8, then the compiler would already be able to recognize some invalid access to array elements at compile time — a[9] would surely be a compile-time error. However, at runtime, when we execute our program, access to a non-existent index position may occur, for example, with constructs like var i = 9; a[i] = 1, when the array is declared as var a: array[8, int]. For catching that type of error, the compiler has to store the fixed array size somewhere and check against that value when an array access by using the subscript operator with a non-constant argument occurs, as the a[i] above. One related remark: Accessing array elements is as fast as ordinary variable access when we use a constant value as an index; that is, a constant literal or a named constant. The reason for this is, that when the index is a constant, then the compiler just knows the exact position of that array element in memory, just as it knows the address of plain variables, so there is no need for address calculations at runtime. Indeed, to access an array element at a specific constant index position, the compiler only needs to add a constant value to the current stack pointer, given that arrays are stored on the stack. To access a constant position in a seq, the compiler would have to add a constant to the base address of the memory block that contains the seq data.

We said that appending elements to sequences is not extremely fast — indeed, it is several times slower than accessing an array element by its index using the subscript operator. So, when we know that our seq will need to contain at least a certain number of elements, it can be more performance-efficient to allocate the seq with this size from the beginning and then fill in the content using the subscript operator, rather than appending all the elements one by one. Here is one example:

We use the newSeq() procedure to initialize the sequence. The content of the square brackets instructs the newSeq() proc to create a sequence with a base type of int, and the number 8 as an argument indicates that the newly created sequence should contain 8 elements, each with the default value of 0. This procedure is what is known as a generic proc, and it requires additional information, specifically, the data type of the elements. Don’t confuse the square brackets in the newSeq[int]() call with the subscript operator a[i] used for array access, as they are completely unrelated. Note that the initialization of the seq above does not restrict its use in any way, we can still use it like an uninitialized seq, that is we can use the add() operator to add more elements, we can insert or delete elements, and all that.

Deleting elements from an array or a sequence can be very slow, particularly when we use the naive approach of moving all the elements located after the element that should be removed one position forward.^[45]

This would maintain the order in the container, so sometimes this is the only solution, but of course, moving all the entries is expensive for large containers. Nim’s standard library provides the delete() function for this order maintaining delete operation. A much faster way to delete an entry in a seq or array is to remove the last entry and replace the one that should be deleted with that last entry. This operation moves the last entry to replace the one that should be deleted, so the order of elements is not maintained. Nim’s standard library provides the del() function for this faster, but order-changing delete operation. Naturally, we should use del() when the order is not important. The delete() and del() functions are actually only available for sequences, as arrays have a fixed size — but in principle, we could do similar operations with arrays as well; we just have to store the actual used size somewhere. ^[46]

Nim slices are objects of type Slice with two fields, a lower bound (a) and an upper bound (b). The system module also defines the HSlice object, called a heterogeneous slice, for which the lower and upper bound can have different data types:

As the Slice and HSlice objects are not built-in types, their names start with capital letters. Slices are not used that often directly, but mostly indirectly with the .. range operator, e.g. to access sub-ranges of strings and other containers.