Programming language: Crystal
License: MIT License
Tags: Misc    
Latest version: v0.12.0

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Anyolite is a Crystal shard which adds a fully functional mruby interpreter to Crystal.


Release ReleaseDate



Anyolite allows for wrapping Crystal classes and functions into mruby with little effort. This way, mruby can be used as a scripting language to Crystal projects, with the major advantage of a similar syntax.

This project is currently in active development, so please report any bugs or missing relevant features.

Useful links for an overview:


  • Bindings to an mruby interpreter
  • Wrapping of nearly arbitrary Crystal classes and methods to mruby
  • Easy syntax without unnecessary boilerplate code
  • Simple system to prevent garbage collector conflicts
  • Support for keyword arguments and default values
  • Objects, arrays, hashes, structs, enums and unions as function arguments and return values are completely valid
  • Ruby methods can be called at runtime as long as all their possible return values are known
  • Ruby closures can be handled as regular variables
  • Methods and constants can be excluded, modified or renamed with annotations
  • Options to compile scripts directly into the executable


You need to have the following programs installed (and in your PATH variable, if you are on Windows):

  • Ruby (for building mruby)
  • Rake (for building the whole project)
  • Bison (for building mruby)
  • Git (for downloading mruby)
  • GCC or Microsoft Visual Studio 19 (for building the object files required for Anyolite, depending on your OS)

Experimental MRI test

If you want to test MRI as implementation, you need these additional programs:

  • Autoconf

Compiling Anyolite for MRI requires setting the environment variable ANYOLITE_CONFIG_PATH to a valid MRI configuration path (like config_files/anyolite_config_mri.json), building the shard and then setting the anyolite_implementation_ruby_3 and use_general_object_format_chars flags for the final compilation.

Support for MRI is still not complete and many problems and errors might be encountered, so mruby is still recommended as the main Ruby implementation for now.

Please report any bugs with MRI, so development can progress smoothly.

Known issues

  • Currently it is only possible to run a single actual Ruby script file
  • UTF-8 function and variable names defined in Crystal can lead to crashes in Ruby
  • Bytecode compilation functions are not available yet (and might never be)
  • Some utility functions from mruby are not available in MRI
  • Gems need to be installed manually after installing Ruby
  • For now, only gcc is supported as compiler


Put this shard as a requirement into your shard.yml project file and then call

shards install

from a terminal or the MSVC Developer Console (on Windows).

Alternatively, you can clone this repository into the lib folder of your project and run

rake build_shard

manually to install the shard without using the crystal shards program.

If you want to use other options for Anyolite, you can set ANYOLITE_CONFIG_PATH to the filename of a JSON config file, which allows for changing multiple options when installing the shard. Possible options are:

  • ANYOLITE_BUILD_PATH - The relative directory in which Anyolite will be built
  • ANYOLITE_RUBY_FORK - The web address of the mruby repository
  • ANYOLITE_RUBY_RELEASE - The release tag of the mruby version to be used
  • ANYOLITE_RUBY_DIR - The relative directory mruby will be installed in
  • ANYOLITE_RUBY_CONFIG - The config file which is used for building mruby
  • ANYOLITE_GLUE_DIR - The directory in which helper function C files are located
  • ANYOLITE_COMPILER - The C compiler used for building Anyolite

How to use

Imagine a Crystal class for a really bad RPG:

module RPGTest
  class Entity
    property hp : Int32

    def initialize(@hp : Int32)

    def damage(diff : Int32)
      @hp -= diff

    def yell(sound : String, loud : Bool = false)
      if loud
        puts "Entity yelled: #{sound.upcase}"
        puts "Entity yelled: #{sound}"

    def absorb_hp_from(other : Entity)
      @hp += other.hp
      other.hp = 0

Now, you want to wrap this class in Ruby. All you need to do is to execute the following code in Crystal (current commit; see documentation page for the version of the latest release):

require "anyolite"

Anyolite::RbInterpreter.create do |rb|
  Anyolite.wrap(rb, RPGTest)


Well, that's it already. The last line in the block calls the following example script:

a = RPGTest::Entity.new(hp: 20)
a.damage(diff: 13)
puts a.hp

b = RPGTest::Entity.new(hp: 10)
a.absorb_hp_from(other: b)
puts a.hp
puts b.hp
b.yell(sound: 'Ouch, you stole my HP!', loud: true)
a.yell(sound: 'Well, take better care of your public attributes!')

The example above gives a good overview over the things you can already do with Anyolite. More features will be added in the future.


Hard limitations

These limitations can not be circumvented using other methods. It might be possible to remove them in future versions, but for now they are potential roadblocks.

  • Currently, Anyolite does not work on Windows due to Crystal compiler bugs
  • Anyolite is only compatible with mruby 3 at the current time

Soft limitations

The limitations here do not have a trivial solution (yet), but with some tricks and tools from Anyolite it should technically be possible to circumvent them (possible solutions are written below each problem). If one of these does definitely not work, but you need them to, please feel free to open an issue.

Procs as arguments are possible, but need special handling

Either annotate the methods using AddBlockArg or StoreBlockArg.

Symbols do not work fully due to their compiletime nature in Crystal

If all symbols are known beforehand, they can be casted from strings.

Arrays, hashes and strings passed from Crystal to Ruby (or vice versa) are immutable

Do not pass the containers directly, but write special access methods.

Only one function with the same name can be wrapped

Overloading works if you specify the argument type as union and avoid illegal calls.

Splat arguments and arbitrary keywords are not possible due to their reliance on symbols

Passing a hash with strings as keys is a workaround.

Keywords will always be given to functions, even if optional (then with default values)

Try to avoid complex function calls in default arguments.

Non-keyword function arguments are always set to their default values before receiving their final values

Same as above.

Default arguments need to be specialized with their full class and module path in order to work

Use the Specialize annotations to change the default values, if needed.

Some union and generic types need to be specialized with their full path

Use the Specialize annotations to specify the full path if necessary.

Private constants trigger errors, which can not be prevented by Anyolite

Use the ExcludeConstant annotation to exclude private constants.

Pointers given to Ruby are weak references and therefore not tracked by the garbage collector

Try to avoid pointers wherever possible, otherwise keep track of the referenced objects.

Why this name?


In short, it is a rare variant of the crystalline mineral called zoisite, with ruby and other crystal shards (of pargasite) embedded.

The term 'anyoli' means 'green' in the Maasai language, thus naming 'anyolite'.


Upcoming releases

Version 0.13.0


  • [ ] Full MRI Ruby as alternative implementation (might be postponed to a later release)
  • * [X] Basic functionality similar to mruby
  • * [X] Calling of script lines
  • * [ ] Bytecode compilation (might not be possible)
  • * [ ] Even more tests
  • * [ ] Copy source in build directory and call compile scripts there
  • * [ ] Find solution for UTF-8 function names leading to crashes
  • * [ ] Find solution for MRI crash at second script file loading
  • [X] AnyolitePointer helper class for accessing pointers
  • [X] Infrastructure to convert script files into bytecode at runtime and compiletime
  • [X] Support for setting and getting instance, class and global variables from Crystal

Breaking changes

  • [X] Changed RClass* to RClassPtr to allow compatibility with MRI
  • [X] Reorganized some macros
  • [X] Changed directory structure
  • [X] Block variables for functions definitions have now an underscore in front of them


  • [X] Option for defaulting to usage of RbValue as data container for regular arguments
  • [X] Alternate build paths are now passed to Anyolite via the environment variable ANYOLITE_BUILD_PATH


  • [X] Error messages for problems when loading scripts or bytecode files


  • [X] Alternate build paths are not recognized properly in implementation files
  • [X] Fixed typo in name of rb_str_to_cstr
  • [X] Fixed inconsistent usage of rb in many functions

Version 1.0.0

This release will mark the first full release of Anyolite, mostly focussed on platform support, more examples and code quality.

Other versions might still come before this, especially for bugfixes, but most of the features for a full release of Anyolite are already implemented.

Platform support

  • [ ] Windows support (currently not supported due to problems with Crystal)
  • [ ] Mac support (might be possible, not tested yet)


  • [ ] Crystal specs for testing
  • [ ] Documentation of all relevant features and wrappers

Configuration options

  • [ ] More configuration options for the Rakefile

Code quality

  • [ ] Convert macro body variables to fresh variables wherever possible
  • [ ] Put macro function arguments as options in a hash
  • [ ] Code cleanup (especially in the macro section)
  • [ ] More compatibility between methods accepting RbRef and RbValue

Future feature ideas (might not actually be possible to implement)

  • [ ] Splat argument and/or arbitrary keyword passing
  • [ ] Support for slices
  • [ ] Classes as argument type
  • [ ] Resolve context even in generic type union arguments
  • [ ] Ability to choose between mruby and regular Ruby
  • [ ] Automatic wrappers for initialize_copy and similar methods
  • [ ] Class inheritance wrapping can be disabled for any class using annotations
  • [ ] General improvement of type resolving
  • [ ] Return values from evaluated script lines

*Note that all licence references and agreements mentioned in the anyolite README section above are relevant to that project's source code only.