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DIY Lang

batteries included, some assembly required

In this tutorial/workshop we'll be implementing our own little language, more or less from scratch.

By the end of the tutorial you will be the proud author of a programming language, and will hopefully better understand how programming languages work on a fundamental level.

What we will be making

We will make a relatively simple, but neat language. We aim for the following features:

  • A handful of data types (integers, booleans and symbols)
  • Variables
  • First class functions with lexical scoping
  • That nice homemade quality feeling

We will not have:

  • A proper type system
  • Error handling
  • Good performance
  • And much, much more

The language should be able to interpret the following code by the time we are done:

(define fact
    ;; Factorial function
    (lambda (n)
        (if (eq n 0)
            1 ; Factorial of 0 is 1
            (* n (fact (- n 1))))))

;; When parsing the file, the last statement is returned
(fact 5)

The syntax is very similar to languages in the Lisp family. If you find the example unfamiliar, you might want to have a look at a more detailed description of the language.


First, clone this repo.

git clone
cd diy-lang

Then, depending on your platform:

  • Mac: Install Python, either from the webpage or using brew. Then run easy_install nose to install nose, the test runner we'll be using.

    Optional: If you are familiar with virtualenv you might want to install nose in a separate pyenv to keep everything tidy.

  • Windows/Linux: Install Python, either from the webpage or your package manager of choice. Then install Pip. Finally, install nose like this: pip install nose.

    Optional: If you are familiar with virtualenv you might want to install nose in a separate pyenv to keep everything tidy.

  • Vagrant: If you have Vagrant installed, an easy way to get going is to use the provided Vagrantfile. Use vagrant up to boot the box, and vagrant ssh to log in. The project folder is synced to /home/vagrant/diy-lang.

Test your setup

Once installed, run nosetests --stop see that everything is working properly. This will run the test suite, stopping at the first failure. Expect something like the following:

$ nosetests --stop
ERROR: TEST 1.1: Parsing a single symbol.
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/nose/", line 197, in runTest
  File "/home/vagrant/diy-lang/tests/", line 15, in test_parse_single_symbol
    assert_equals('foo', parse('foo'))
  File "/home/vagrant/diy-lang/diylang/", line 17, in parse
    raise NotImplementedError("DIY")
NotImplementedError: DIY

Ran 1 test in 0.034s

FAILED (errors=1)

A few tips

Take the time to consider the following points before we get going:

  • Keep things simple

    Don't make things more complicated than they need to be. The tests should hopefully guide you every step of the way.

  • Read the test descriptions

    Each test has a small text describing what you are going to implement and why. Reading these should make things easier, and you might end up learning more.

  • Use the provided functions

    Some of the more boring details are already taken care of. Take the time to look at the functions provided in, and the various imports in files where you need to do some work.

  • The Python cheat sheet in

    Unless you're fluent in Python, there should be some helpful pointers in the Python cheat sheet. Also, if Python is very new to you, the Python tutorial might prove helpful.

  • Description of your language

    Read a description of the language you are going to make in

Get started!

The workshop is split up into eight parts. Each consist of an introduction, and a bunch of unit tests which it is your task to make run. When all the tests run, you'll have implemented that part of the language.

Have fun!


A hands-on, test driven guide to implementing a simple programming language




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