A structured, dynamic, general-purpose language. http://myst-lang.org


A structured, dynamic, general-purpose language.

deftype List
  def contains(element)
      ->(<element>) { break true }
      ->(_)         { false }

[1, 2, 3].contains(2) #=> true

Some of the high-level features include:

  • Pattern-matching everywhere. Assignments, method parameters, rescue clauses, etc.
  • Multiple-clause functions. All functions can define multiple clauses to adapt functionality based on inputs.
  • Value interpolations. Interpolate any value anywhere (even in method parameters) with the <> syntax.
  • Soft typing. Optional type annotations help control functionality without cluttering your code with conditionals.
  • Raise anything. Any value can be raised as an Exception and pattern matched in a rescue block.


NOTE: Due to Crystal's current limitations with compiling on Windows, Myst only works on macOS and Linux systems.

The recommended method of installing Myst is with mtenv, the official version manager for the Myst language. It is available here and has installation instructions available in the README.

For now, you will need to have Crystal installed to be able to install Myst. See Crystal's installation instructions for how to get started. Myst currently runs on Crystal 0.27.0.

Once Crystal and mtenv are installed, installing Myst is as simple as running mtenv install:

# Make sure mtenv is properly set up
mtenv setup
# Install v0.6.2 of Myst
mtenv install v0.6.2
# Make it the active version
mtenv use v0.6.2

With that, myst should now be installed and ready to go!

Help with improving these installation instructions, making pre-built binaries, and/or managing releases would be greatly appreciated :)

Get Involved

If you have an idea for a new feature or find a bug in Myst, please file an issue for it!. Using the language and finding bugs are the best ways to help Myst improve. Any and all help here is appreciated, even if that just means trying out the language for a day.

If you just want to get involved in the community, come hang out in our Discord server!. We're a pretty small community, so there's plenty of room for anyone that would like to hang out, even if it has nothing to do with Myst!

When I can, I try to label issues with help wanted or good first issue. help wanted is for issues that I'd really like external input on, while good first issue is for issues that can be implemented without too much knowledge of how the lexer/parser/interpreter works. On these issues, I try to explain as much as possible about what the solution looks like, including files that will need editing and/or methods that need implementing/changing. I hope that helps!

If you'd like to tackle something, but don't know where to start, please let me know! I'd love to help you get involved, so feel free to ask in the discord server or message me directly (faulty#7958 on discord, or email also works) and I'll do my best to get you up and running.

The Basics

If you would like to contribute to Myst's development, just:

  1. Fork it (https://github.com/myst-lang/myst/fork)
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create a new Pull Request (https://github.com/myst-lang/myst/pull/new)

Owning an issue

If you have a specific issue that you'd like to tackle, be sure to add a comment saying you're working on it so that everyone is aware! (currently, github doesn't allow for assigning issues to new contributers :/)

Also, "ownership" is not binding. It's just a way of saying "hey, I think I can work on this!". If you get stuck or need help moving forward, feel free to ask for help either on the issue itself, or in the discord server.

Most importantly, don't feel bad if you bite off more than you can chew. Issues can easily end up being far more complex than they appear at the start, especially on a project of this size. But don't give up! It's always hard to get started on an existing project, but I want to help and make it as easy as possible wherever I can!


All notable changes to this project will be documented in this file.

The format is based on Keep a Changelog and this project adheres to Semantic Versioning.



  • Many additions to the standard library (See #57. Thanks @bmulvihill, @minirop, @atuley, and @zkayser). Integer, Float, and Enumerable received big buffs, among other things.
  • Added a Time type and Time#now to get the current system time (see #85. Thanks, @bmulvihill).
  • Added the extend keyword for adding Module ancestors to the static scope of a Type (see #77. Thanks, @zkayser).
  • Added support for the Splat operator (*) for deconstructing objects into Lists of values (see #71).
  • Added support for Unary operators Not (!) and Negation (-) (see #52, #46. Thanks, @rainbru).
  • Allow any functor to be "captured" with the & unary operator (see #50).
  • Added support for anonymous functions using the fn keyword (see #47).
  • Object instantiations can accept captured functions as the block parameter (See #91). This keeps parity with regular method calls.
  • Added __FILE__, __LINE__, and __DIR__ magic constants (see #90, #45. Thanks, @bmulvihill).

Bug Fixes

  • Properly maintain local scope overrides after rescuing an error (see #95).
  • Properly restore the value self after rescuing an error (see #65).
  • Fixed a bug with List#== where lists were incorrectly considered equal (see #89. Thanks, @bmulvihill).
  • Open angle brackets in Strings no longer cause a ParseError (see #80. Thanks, @bmulvihill).
  • Accessing/assigning non-existent elements in a List no longer raises an error (see #72. Thanks, @bmulvihill).


  • Adding parentheses after a Var will always force it to be a Call, even with no arguments (see #54. Thanks, @bmulvihill).


  • Upgraded to Crystal 0.24.1 (see #99). This should improve the experience of developing Myst, particularly on macOS.
  • Fixed the fibonnaci example code to use the correct sequence numbers in the comments (see #55. Thanks, @minirop).



  • Interpolate any expression into a string using <(...)> (#33)
  • Add operational assignments (e.g., ||= and +=) for shorter, cleaner expressions (#30)
  • Allow operators as method names (#29)
  • Query and bang (? and !) methods (#28)
  • Warning messages when referencing Underscore variables (#34. Thanks, @rainbru!)
  • self returns the interpreter's current self value (#40)
  • Basic Spec library for making assertions in Myst code (#39)

Bug fixes

  • Fixed Constant lookup from instances to also check the static scope (#38)


  • Better linux installation instructions (#32. Thanks, @rainbru!)


Initial release of the Myst language!

Github statistic:
  • 79
  • 14
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  • 16
  • 6 days ago


MIT License