Nim with Dennis Felsing

Nim is a young and exciting imperative programming language that is nearing its 1.0 release. My main motivation for using Nim is its performance / productivity ratio and the joy of programming in Nim. In this guide I’m going to show you how I start a Nim project.

For this purpose we will write a small interpreter for the brainfuck language. While Nim is a practical language with many interesting features, brainfuck is the opposite: It’s impractical to write in and its features consist of 8 single-character commands. Still, brainfuck is great for us, since its extreme simplicity makes it easy to write an interpreter for it. Later we will even write a high-performance compiler that transforms brainfuck programs into Nim at compile time. We will put all of this into a nimble package and publish it online.


Installing Nim is straightforward, you can follow the official instructions. Binaries for Windows are provided. On other operating systems you can run the script to compile the generated C code, which should take less than 1 minute on a modern system.

This brings us to the first interesting fact about Nim: It compiles to C primarily (C++, ObjectiveC and even JavaScript as well) and then uses the highly optimizing C compiler of your choice to generate the actual program. You get to benefit from the mature C ecosystem for free.

If you opt for bootstrapping the Nim compiler, which is written exclusively in Nim itself, you get to witness the compiler build itself with a few simple steps (in less than 2 minutes):

$ git clone
$ cd Nim
$ git clone --depth 1
$ cd csources && sh
$ cd ..
$ bin/nim c koch
$ ./koch boot -d:release

This way you get a development version of Nim. To keep it up to date these two steps should be enough:

$ git pull
$ ./koch boot -d:release

If you haven’t done so already, now is a good time to install git as well. Most nimble packages are available on github, so we will need git to get them. On a Debian based distribution (like Ubuntu) we can install it like this:

$ sudo apt-get install git

After you’ve finished the installation, you should add the nim binary to your path. If you use bash, this is what to do:

$ echo 'export PATH=$PATH:$your_install_dir/bin' >> ~/.profile
$ source ~/.profile
$ nim
Nim Compiler Version 0.10.2 (2014-12-29) [Linux: amd64]
Copyright (c) 2006-2014 by Andreas Rumpf

    nim command [options] [projectfile] [arguments]

  compile, c                compile project with default code generator (C)
  doc                       generate the documentation for inputfile
  doc2                      generate the documentation for the whole project
  i                         start Nim in interactive mode (limited)

If nim reports its version and usage, we’re good to continue. Now the modules from Nim’s standard library are just an import away. All other packages can be retrieved with nimble, Nim’s package manager. Let’s follow the simple installation instructions. Again, for Windows a prebuilt archive is available, while building from source is quite comfortable as well:

$ git clone
$ cd nimble
$ nim c -r src/nimble install

Nimble’s binary directory wants to be added to your path as well:

$ echo 'export PATH=$PATH:$HOME/.nimble/bin' >> ~/.profile
$ source ~/.profile
$ nimble update
Downloading package list from

Now we can browse the available nimble packages or search for them on the command line:

$ nimble search docopt
  url:         git:// (git)
  tags:        commandline, arguments, parsing, library
  description: Command-line args parser based on Usage message
  license:     MIT

Let’s install this nice docopt library we found, maybe we’ll need it later:

$ nimble install docopt
docopt installed successfully.

Notice how quickly the library is installed (less than 1 second for me). This is another nice effect of Nim. Basically the source code of the library is just downloaded, nothing resembling a shared library is compiled. Instead the library will simply be compiled statically into our program once we use it.

There is Nim editor support for most of the popular editors out there, like Emacs (nim-mode), Vim (nimrod.vim, my choice) and Sublime (NimLime). For the scope of this guide any text editor will do.

Project Setup

Now we’re ready to get our project started:

$ mkdir brainfuck
$ cd brainfuck

First step: To get Hello World on the terminal, we create a hello.nim with the following content:

echo "Hello World"

We compile the code and run it, first in two separate steps:

$ nim c hello
$ ./hello
Hello World

Then in a single step, by instructing the Nim compiler to conveniently run the resulting binary immediately after creating it:

$ nim c -r hello
Hello World

Let’s make our code do something slightly more complicated, that should take a bit longer to run:

var x = 0
for i in 1 .. 100_000_000:
  inc x # increase x, this is a comment btw

echo "Hello World ", x

Now we’re initializing the variable x to 0 and increase it by 1 a whole 100 million times. Try to compile and run it again. Notice how long it takes to run now. Is Nim’s performance that abysmal? Of course not, quite the opposite! We’re just currently building the binary in full debug mode, adding checks for integer overflows, array out of bounds and much more, as well as not optimizing the binary at all. The -d:release option allows us to switch into release mode, giving us full speed:

$ nim c hello
$ time ./hello
Hello World 100000000
./hello  2.01s user 0.00s system 99% cpu 2.013 total
$ nim -d:release c hello
$ time ./hello
Hello World 100000000
./hello  0.00s user 0.00s system 74% cpu 0.002 total

That’s a bit too fast actually. The C compiler optimized away the entire for loop. Oops.

To start a new project nimble init can generate a basic package config file:

$ nimble init brainfuck

The newly created brainfuck.nimble should look like this:

name          = "brainfuck"
version       = "0.1.0"
author        = "Anonymous"
description   = "New Nimble project for Nim"
license       = "BSD"

Requires: "nim >= 0.10.0"

Let’s add the actual author, a description, as well as the requirement for docopt, as described in nimble’s developers info. Most importantly, let’s set the binary we want to create:

name          = "brainfuck"
version       = "0.1.0"
author        = "The 'How I Start Nim' Team"
description   = "A brainfuck interpreter"
license       = "MIT"

bin           = "brainfuck"

Requires: "nim >= 0.10.0, docopt >= 0.1.0"

Since we have git installed already, we’ll want to keep revisions of our source code and may want to publish them online at some point, let’s initialize a git repository:

$ git init
$ git add hello.nim brainfuck.nimble .gitignore

Where I just initialized the .gitignore file to this:


We tell git to ignore vim’s swap files, as well as nimcache directories that contain the generated C code for our project. Check it out if you’re curious how Nim compiles to C.

To see what nimble can do, let’s initialize brainfuck.nim, our main program:

echo "Welcome to brainfuck"

We could compile it as we did before for hello.nim, but since we already set our package up to include the brainfuck binary, let’s make nimble do the work:

$ nimble build
Looking for docopt (>= 0.1.0)...
Dependency already satisfied.
Building brainfuck/brainfuck using c backend...
$ ./brainfuck
Welcome to brainfuck

nimble install can be used to install the binary on our system, so that we can run it from anywhere:

$ nimble install
brainfuck installed successfully.
$ brainfuck
Welcome to brainfuck

This is great for when the program works, but nimble build actually does a release build for us. That takes a bit longer than a debug build, and leaves out the checks which are so important during development, so nim c -r brainfuck will be a better fit for now. Feel free to execute our program quite often during development to get a feeling for how everything works.


While programming Nim’s documentation comes in handy. If you don’t know where to find what yet, there’s a documentation index, in which you can search.

Let’s start developing our interpreter by changing the brainfuck.nim file:

import os

First we import the os module, so that we can read command line arguments.

let code = if paramCount() > 0: readFile paramStr(1)
           else: readAll stdin

paramCount() tells us about the number of command line arguments passed to the application. If we get a command line argument, we assume it’s a filename, and read it in directly with readFile paramStr(1). Otherwise we read everything from the standard input. In both cases, the result is stored in the code variable, which has been declared immutable with the let keyword.

To see if this works, we can echo the code:

echo code

And try it out:

$ nim c -r brainfuck
Welcome to brainfuck
I'm entering something here and it is printed back later!
I'm entering something here and it is printed back later!

After you’ve entered your “code” finish up with a newline and ctrl-d. Or you can pass in a filename, everything after nim c -r brainfuck is passed as command line arguments to the resulting binary:

$ nim c -r brainfuck .gitignore
Welcome to brainfuck

On we go:

  tape = newSeq[char]()
  codePos = 0
  tapePos = 0

We declare a few variables that we’ll need. We have to remember our current position in the code string (codePos) as well as on the tape (tapePos). Brainfuck works on an infinitely growing tape, which we represent as a seq of chars. Sequences are Nim’s dynamic length arrays, other than with newSeq they can also be initialized using var x = @[1, 2, 3].

Let’s take a moment to appreciate that we don’t have to specify the type of our variables, it is automatically inferred. If we wanted to be more explicit, we could do so:

  tape: seq[char] = newSeq[char]()
  codePos: int = 0
  tapePos: int = 0

Next we write a small procedure, and call it immediately afterwards:

proc run(skip = false): bool =
  echo "codePos: ", codePos, " tapePos: ", tapePos

discard run()

There are a few things to note here:

  • We pass a skip parameter, initialized to false.
  • Obviously the parameter must be of type bool then.
  • The return type is bool as well, but we return nothing? Every result is initialized to binary 0 by default, meaning we return false.
  • We can use the implicit result variable in every proc with a result and set result = true.
  • Control flow can be changed by using return true to return immediately.
  • We have to explicitly discard the returned bool value when calling run(). Otherwise the compiler complains with brainfuck.nim(16, 3) Error: value of type 'bool' has to be discarded. This is to prevent us from forgetting to handle the result.

Before we continue, let’s think about the way brainfuck works. Some of this may look familiar if you’ve encountered Turing machines before. We have an input string code and a tape of chars that can grow infinitely in one direction. These are the 8 commands that can occur in the input string, every other character is ignored:

Op Meaning Nim equivalent
> move right on tape inc tapePos
< move left on tape dec tapePos
+ increment value on tape inc tape[tapePos]
- decrement value on tape dec tape[tapePos]
. output value on tape stdout.write tape[tapePos]
, input value to tape tape[tapePos] = stdin.readChar
[ if value on tape is \0, jump forward to command after matching ]
] if value on tape is not \0, jump back to command after matching [

With this alone, brainfuck is one of the simplest Turing complete programming languages.

The first 6 commands can easily be converted into a case distinction in Nim:

proc run(skip = false): bool =
  case code[codePos]
  of '+': inc tape[tapePos]
  of '-': dec tape[tapePos]
  of '>': inc tapePos
  of '<': dec tapePos
  of '.': stdout.write tape[tapePos]
  of ',': tape[tapePos] = stdin.readChar
  else: discard

We are handling a single character from the input so far, let’s make this a loop to handle them all:

proc run(skip = false): bool =
  while tapePos >= 0 and codePos < code.len:
    case code[codePos]
    of '+': inc tape[tapePos]
    of '-': dec tape[tapePos]
    of '>': inc tapePos
    of '<': dec tapePos
    of '.': stdout.write tape[tapePos]
    of ',': tape[tapePos] = stdin.readChar
    else: discard

    inc codePos

Let’s try a simple program, like this:

$ echo ">+" | nim -r c brainfuck
Welcome to brainfuck
Traceback (most recent call last)
brainfuck.nim(26)        brainfuck
brainfuck.nim(16)        run
Error: unhandled exception: index out of bounds [IndexError]
Error: execution of an external program failed

What a shocking result, our code crashes! What did we do wrong? The tape is supposed to grow infinitely, but we haven’t increased its size at all! That’s an easy fix right above the case:

    if tapePos >= tape.len:
      tape.add '\0'

The last 2 commands, [ and ] form a simple loop. We can encode them into our code as well:

proc run(skip = false): bool =
  while tapePos >= 0 and codePos < code.len:
    if tapePos >= tape.len:
      tape.add '\0'

    if code[codePos] == '[':
      inc codePos
      let oldPos = codePos
      while run(tape[tapePos] == '\0'):
        codePos = oldPos
    elif code[codePos] == ']':
      return tape[tapePos] != '\0'
    elif not skip:
      case code[codePos]
      of '+': inc tape[tapePos]
      of '-': dec tape[tapePos]
      of '>': inc tapePos
      of '<': dec tapePos
      of '.': stdout.write tape[tapePos]
      of ',': tape[tapePos] = stdin.readChar
      else: discard

    inc codePos

If we encounter a [ we recursively call the run function itself, looping until the corresponding ] lands on a tapePos that doesn’t have \0 on the tape.

If you’re on Nim 0.11 or a newer version, you’ll run into another problem: The inc and dec procs for chars have overflow (and underflow) checks. This means that when we have the character \0 and decrement it, we end up with a runtime error! Instead, in brainfuck, we want to wrap around and get \255 instead. We cold use a uint8 instead of a char, because unsigned ints wrap around in Nim. But then we have to convert that uint8 to a char sometimes and the other way around. A more convenient way is to define our own, non-overflow-checking xinc and xdec procs:

{.push overflowchecks: off.}
proc xinc(c: var char) = inc c
proc xdec(c: var char) = dec c

We use Nim’s pragma system to disable overflow checks just for this part of the code, not touching the settings for the rest of the program. Now of course two cases have to change:

      of '+': xinc tape[tapePos]
      of '-': xdec tape[tapePos]

And that’s it. We have a working brainfuck interpreter now. To test it, we create an examples directory containing these 3 files: helloworld.b, rot13.b, mandelbrot.b.

$ nim -r c brainfuck examples/helloworld.b
Welcome to brainfuck
Hello World!
$ ./brainfuck examples/rot13.b
Welcome to brainfuck
You can enter anything here!
Lbh pna ragre nalguvat urer!
$ ./brainfuck examples/mandelbrot.b

With the last one you will notice how slow our interpreter is. Compiling with -d:release gives a nice speedup, but still takes about 90 seconds on my machine to draw the Mandelbrot set. To achieve a great speedup, later on we will compile brainfuck to Nim instead of interpreting it. Nim’s metaprogramming capabilities are perfect for this.

But let’s keep it simple for now. Our interpreter is working, now we can turn our work into a reusable library. All we have to do is surround our code with a big proc:

proc interpret*(code: string) =
    tape = newSeq[char]()
    codePos = 0
    tapePos = 0

  proc run(skip = false): bool =

  discard run()

when isMainModule:
  import os

  echo "Welcome to brainfuck"

  let code = if paramCount() > 0: readFile paramStr(1)
             else: readAll stdin

  interpret code

Note that we also added a * to the proc, which indicates that it is exported and can be accessed from outside of our module. Everything else is hidden.

At the end of the file we still kept the code for our binary. when isMainModule ensures that this code is only compiled when this module is the main one. After a quick nimble install our brainfuck library can be used from anywhere on your system, just like this:

import brainfuck
interpret "++++++++[>++++[>++>+++>+++>+<<<<-]>+>+>->>+[<]<-]>>.>---.+++++++..+++.>>.<-.<.+++.------.--------.>>+.>++."

Looking good! At this point we could share the code with others already, but let’s add some documentation first:

proc interpret*(code: string) =
  ## Interprets the brainfuck `code` string, reading from stdin and writing to
  ## stdout.

nim doc brainfuck builds the documentation, which you can see online in its full glory.


As I said before, our interpreter is still pretty slow for the mandelbrot program. Let’s write a procedure that creates Nim code AST at compile time instead:

import macros

proc compile(code: string): PNimrodNode {.compiletime.} =
  var stmts = @[newStmtList()]

  template addStmt(text): stmt =
    stmts[stmts.high].add parseStmt(text)

  addStmt "var tape: array[1_000_000, char]"
  addStmt "var tapePos = 0"

  for c in code:
    case c
    of '+': addStmt "xinc tape[tapePos]"
    of '-': addStmt "xdec tape[tapePos]"
    of '>': addStmt "inc tapePos"
    of '<': addStmt "dec tapePos"
    of '.': addStmt "stdout.write tape[tapePos]"
    of ',': addStmt "tape[tapePos] = stdin.readChar"
    of '[': stmts.add newStmtList()
    of ']':
      var loop = newNimNode(nnkWhileStmt)
      loop.add parseExpr("tape[tapePos] != '\\0'")
      loop.add stmts.pop
      stmts[stmts.high].add loop
    else: discard

  result = stmts[0]
  echo result.repr

The template addStmt is just there to reduce boilerplate. We could also explicitly write the same operation at each position that currently uses addStmt. (And that’s exactly what a template does!) parseStmt turns a piece of Nim code from a string into its corresponding AST, which we store in a list.

Most of the code is similar to the interpreter, except we’re not executing the code now, but generating it, and adding it to a list of statements. [ and ] are more complicated: They get translated into a while loop surrounding the code inbetween.

We’re cheating a bit here because we use a fixed size tape now and don’t check for under- and overflows anymore. This is mainly for the sake of simplicity. To see what this code does, the last line, namely echo result.repr prints the Nim code we generated.

Try it out by calling it inside a static block, which forces execution at compile time:

  discard compile "+>+[-]>,."

During compilation the generated code is printed:

var tape: array[1000000, char]
var codePos = 0
var tapePos = 0
xinc tape[tapePos]
inc tapePos
xinc tape[tapePos]
while tape[tapePos] != '\0':
  xdec tape[tapePos]
inc tapePos
tape[tapePos] = stdin.readChar
stdout.write tape[tapePos]

Generally useful for writing macros is the dumpTree macro, which prints the AST of a piece of code (actual one, not as a string), for example:

import macros

  while tape[tapePos] != '\0':
    inc tapePos

This shows us the following Tree:

      Ident !"!="
        Ident !"tape"
        Ident !"tapePos"
      CharLit 0
        Ident !"inc"
        Ident !"tapePos"

That’s how I knew that we would need a StmtList, for example. When you do metaprogramming in Nim, it’s generally a good idea to use dumpTree and print out the AST of the code you want to generate.

Macros can be used to insert the generated code into a program directly:

macro compileString*(code: string): stmt =
  ## Compiles the brainfuck `code` string into Nim code that reads from stdin
  ## and writes to stdout.
  compile code.strval

macro compileFile*(filename: string): stmt =
  ## Compiles the brainfuck code read from `filename` at compile time into Nim
  ## code that reads from stdin and writes to stdout.
  compile staticRead(filename.strval)

We can now compile the mandelbrot program into Nim easily:

proc mandelbrot = compileFile "examples/mandelbrot.b"


Compiling with full optimizations takes quite long now (about 4 seconds), because the mandelbrot program is huge and GCC needs some time to optimize it. In return the program runs in just 1 second:

$ nim -d:release c brainfuck
$ ./brainfuck

Compiler settings

By default Nim compiles its intermediate C code with GCC, but clang usually compiles faster and may even yield more efficient code. It’s always worth a try. To compile once with clang, use nim -d:release --cc:clang c hello. If you want to keep compiling hello.nim with clang, create a hello.nim.cfg file with the content cc = clang. To change the default backend compiler, edit config/nim.cfg in Nim’s directory.

While we’re talking about changing default compiler options. The Nim compiler is quite talky at times, which can be disabled by setting hints = off in the Nim compiler’s config/nim.cfg. One of the more unexpected compiler warnings even warns you if you use l (lowercase L) as an identifier, because it may look similar to 1 (one):

a.nim(1, 4) Warning: 'l' should not be used as an identifier; may look like '1' (one) [SmallLshouldNotBeUsed]

If you’re not a fan of this, a simple warning[SmallLshouldNotBeUsed] = off suffices to make the compiler shut up.

Another advantage of Nim is that we can use debuggers with C support, like GDB. Simply compile your program with nim c --linedir:on --debuginfo c hello and gdb ./hello can be used to debug your program.

Command line argument parsing

So far we’ve been parsing the command line argument by hand. Since we already installed the docopt.nim library before, we can use it now:

when isMainModule:
  import docopt, tables, strutils

  proc mandelbrot = compileFile("examples/mandelbrot.b")

  let doc = """

  brainfuck mandelbrot
  brainfuck interpret [<file.b>]
  brainfuck (-h | --help)
  brainfuck (-v | --version)

  -h --help     Show this screen.
  -v --version  Show version.

  let args = docopt(doc, version = "brainfuck 1.0")

  if args["mandelbrot"]:

  elif args["interpret"]:
    let code = if args["<file.b>"]: readFile($args["<file.b>"])
               else: readAll stdin


The nice thing about docopt is that the documentation functions as the specification. Pretty simple to use:

$ nimble install
brainfuck installed successfully.
$ brainfuck -h

  brainfuck mandelbrot
  brainfuck interpret [<file.b>]
  brainfuck (-h | --help)
  brainfuck (-v | --version)

  -h --help     Show this screen.
  -v --version  Show version.
$ brainfuck interpret examples/helloworld.b
Hello World!


Since our project is growing, we move the main source code into a src directory and add a tests directory, which we will soon need, resulting in a final directory structure like this:

$ tree
├── brainfuck.nimble
├── examples
│   ├── helloworld.b
│   ├── mandelbrot.b
│   └── rot13.b
├── license.txt
├── src
│   └── brainfuck.nim
└── tests
    ├── all.nim
    ├── compile.nim
    ├── interpret.nim
    └── nim.cfg

This also requires us to change the nimble file:

srcDir = "src"
bin    = "brainfuck"

To improve reusability of our code, we turn to refactoring it. The main concern is that we always read from stdin and write to stdout.

Instead of accepting just a code: string as its parameter, we extend the interpret procedure to also receive an input and output stream. This uses the streams module that provides FileStreams and StringStreams:

## :Author: Dennis Felsing
## This module implements an interpreter for the brainfuck programming language
## as well as a compiler of brainfuck into efficient Nim code.
## Example:
## .. code:: nim
##   import brainfuck, streams
##   interpret("++++++++[>++++[>++>+++>+++>+<<<<-]>+>+>->>+[<]<-]>>.>---.+++++++..+++.>>.<-.<.+++.------.--------.>>+.>++.")
##   # Prints "Hello World!"
##   proc mandelbrot = compileFile("examples/mandelbrot.b")
##   mandelbrot() # Draws a mandelbrot set

import streams

proc interpret*(code: string; input, output: Stream) =
  ## Interprets the brainfuck `code` string, reading from `input` and writing
  ## to `output`.
  ## Example:
  ## .. code:: nim
  ##   var inpStream = newStringStream("Hello World!\n")
  ##   var outStream = newFileStream(stdout)
  ##   interpret(readFile("examples/rot13.b"), inpStream, outStream)

I’ve also added some module wide documentation, including example code for how our library can be used. Take a look at the resulting documentation.

Most of the code stays the same, except the handling of brainfuck operations . and ,, which now use output instead of stdout and input instead of stdin:

        of '.': output.write tape[tapePos]
        of ',': tape[tapePos] = input.readCharEOF

What is this strange readCharEOF doing there instead of readChar? On many systems EOF (end of file) means -1. Our brainfuck programs actively use this. This means our brainfuck programs might actually not run on all systems. Meanwhile the streams module strives to be platform independent, so it returns a 0 if we have reached EOF. We use readCharEOF to convert this into a -1 for brainfuck explicitly:

proc readCharEOF*(input: Stream): char =
  result = input.readChar
  if result == '\0': # Streams return 0 for EOF
    result = '\255'  # BF assumes EOF to be -1

At this point you may notice that the order of identifier declarations matters in Nim. If you declare readCharEOF below interpret, you can not use it in interpret. I personally try to adhere to this, as it creates a hierarchy from simple code to more complex code in each module. If you still want to circumvent this , split declaration and definition of readCharEOF by adding this declaration above interpret:

proc readCharEOF*(input: Stream): char

The code to use the interpreter as conveniently as before is pretty simple:

proc interpret*(code, input: string): string =
  ## Interprets the brainfuck `code` string, reading from `input` and returning
  ## the result directly.
  var outStream = newStringStream()
  interpret(code, input.newStringStream, outStream)
  result =

proc interpret*(code: string) =
  ## Interprets the brainfuck `code` string, reading from stdin and writing to
  ## stdout.
  interpret(code, stdin.newFileStream, stdout.newFileStream)

Now the interpret procedure can be used to return a string. This will be important for testing later:

let res = interpret(readFile("examples/rot13.b"), "Hello World!\n")
interpret(readFile("examples/rot13.b")) # with stdout

For the compiler the cleanup is a bit more complicated. First we have to take the input and output as strings, so that the user of this proc can use any stream they want:

proc compile(code, input, output: string): PNimrodNode {.compiletime.} =

Two additional statements are necessary to initialize the input and output streams to the passed strings:

  addStmt "var inpStream = " & input
  addStmt "var outStream = " & output

Of course now we have to use outStream and inpStream instead of stdout and stdin, as well as readCharEOF instead of readChar. Note that we can directly reuse the readCharEOF procedure from the interpreter, no need to duplicate code:

    of '.': addStmt "outStream.write tape[tapePos]"
    of ',': addStmt "tape[tapePos] = inpStream.readCharEOF"

We also add a statement that will abort compilation with a nice error message if the user of our library uses it wrongly:

  addStmt """
    when not compiles(newStringStream()):
        quit("Error: Import the streams module to compile brainfuck code", 1)

To connect the new compile procedure to a compileFile macro that uses stdout and stdin again, we can write:

macro compileFile*(filename: string): stmt =
    "stdin.newFileStream", "stdout.newFileStream")

To read from an input string and write back to an output string:

macro compileFile*(filename: string; input, output: expr): stmt =
  result = compile(staticRead(filename.strval),
    "newStringStream(" & $input & ")", "newStringStream()")
  result.add parseStmt($output & " =")

This unwieldy code allows us to write a compiled rot13 procedure like this, connecting input string and result to the compiled program:

proc rot13(input: string): string =
  compileFile("../examples/rot13.b", input, result)
echo rot13("Hello World!\n")

I did the same for compileString for convenience. You can check out the full code of brainfuck.nim on Github.


There are two main ways of testing code in Nim that you will run across. For small pieces of code you can simply use asserts inside a when isMainModule block at the end of the file. This ensures that the testing code will not be executed when the module is used as a library.

Regular assertions can be turned off in Nim with --assertions:off, which is automatically set when we compile a release build. For this reason instead of assert we use doAssert, which will not be optimized away even in release builds. You will find tests like this at the end of many of the standard library’s modules:

when isMainModule:
  doAssert align("abc", 4) == " abc"
  doAssert align("a", 0) == "a"
  doAssert align("1232", 6) == " 1232"
  doAssert align("1232", 6, '#') == "##1232"

For a bigger project the unittest module comes in handy.

We split up the tests into 3 files in the tests/ directory:

tests/interpret.nim tests the interpreter. We define a new test suite, containing two testers, each checking the resulting strings:

import unittest, brainfuck

suite "brainfuck interpreter":
  test "interpret helloworld":
    let helloworld = readFile("examples/helloworld.b")
    check interpret(helloworld, input = "") == "Hello World!\n"

  test "interpret rot13":
    let rot13 = readFile("examples/rot13.b")
    let conv = interpret(rot13, "How I Start\n")
    check conv == "Ubj V Fgneg\n"
    check interpret(rot13, conv) == "How I Start\n"

Similarly for tests/compile.nim to test our compiler:

import unittest, brainfuck, streams

suite "brainfuck compiler":
  test "compile helloworld":
    proc helloworld: string =
      compileFile("../examples/helloworld.b", "", result)
    check helloworld() == "Hello World!\n"

  test "compile rot13":
    proc rot13(input: string): string =
      compileFile("../examples/rot13.b", input, result)
    let conv = rot13("How I Start\n")
    check conv == "Ubj V Fgneg\n"
    check rot13(conv) == "How I Start\n"

Note how we have to read the examples from ../examples/ with the compiler, instead of examples/ with the interpreter. The reason for this is that the compiler’s staticRead opens the files relative to the location of our file, which resides in tests/.

To combine both tests we create a tests/all.nim that only imports and thereby automatically runs all available test suites:

import interpret, compile

For convenience we also create a tests/nim.cfg that sets some reasonable compiler options for testing and debugging:

# $projectPath is tests/
path = "$projectPath/../src"
hints = off
linedir = on
stacktrace = on
linetrace = on

Finally, to compile and run our tests:

$ nim c -r tests/all
[OK] interpret helloworld
[OK] interpret rot13
[OK] compile helloworld
[OK] compile rot13

Great success, our library works! With this we have a fully fledged library, binary and testing framework.

Time to publish everything on Github and submit a pull request to have brainfuck included in the nimble packages. Once the package is accepted you can find it in the official list and use nimble to search for it and install it:

$ nimble search brainfuck
  url: (git)
  tags:        library, binary, app, interpreter, compiler, language
  description: A brainfuck interpreter and compiler
  license:     MIT
$ nimble install brainfuck

Continuous Integration

CircleCI can be used with Nim for continuous integration, so that our tests are compiled and run whenever a new commit is pushed to Github. Since CircleCI does not know about Nim itself, we have to teach it how to bootstrap the compiler:

    - |
        if [ ! -x ~/nim/bin/nim ]; then
          git clone -b devel --depth 1 git:// ~/nim/
          git clone -b devel --depth 1 git:// ~/nim/csources/
          cd ~/nim/csources; sh; cd ..; rm -rf csources
          bin/nim c koch; ./koch boot -d:release
          ln -fs ~/nim/bin/nim ~/bin/nim
          cd ~/nim; git fetch origin
          git merge FETCH_HEAD | grep "Already up-to-date" || (bin/nim c koch; ./koch boot -d:release)

    - "~/bin/"
    - "~/nim/"

This automatically keeps the compiler up to date. If you want to use the most recently released version of Nim instead of the development build, use the master branch instead of devel in the two git clone calls. Running the tests is straightfoward now:

    - nim c -r tests/all

If your project just needs to compile and run tests/all, then you can drop in this circle.yml directly. Otherwise, the test: section is the only part you will need to change.

The build status badge Build Status can be added to the like this:

# Brainfuck for Nim [![Build Status](](

See the Github page again for the final result and the CircleCI page for the actual builds.


This is the end of our tour through the Nim ecosystem, I hope you enjoyed it and found it as interesting as it was for me to write it.

If you still want to learn more about Nim, I have recently written about what is special about Nim and what makes Nim practical, and have an extensive collection of small programs.

If you’re interested in a more traditional start into Nim, the official tutorial and Nim by Example can guide you.

The Nim community is very welcoming and helpful. Thanks to everyone who had suggestions and found bugs in this document, especially Flaviu Tamas, Andreas Rumpf and Dominik Picheta.

Dennis Felsing


Active Nim contributor and Master's candidate at KIT

Dennis is an active contributor to the Nim language while working on his Master's thesis at KIT. There he worked on research developing a new method for Regression Verification and teaching programming paradigms (Haskell, lambda calculus, type inference, Prolog, Scala, etc.). He also develops and runs DDNet, a unique cooperative 2D game.