Yadriggy (mistletoe in English) is a library for building a domain-specific language (DSL) embedded in Ruby. It was developed for a particular kind of embedded DSLs. These DSLs borrow the syntax from the host language, Ruby, and the code written in the DSLs is embedded in normal Ruby code. However, the execution of the DSL code is independent of Ruby. Its semantics can be totally different from Ruby and the code can be run out of the Ruby VM. These DSLs look like Ruby but they are different languages except their syntax. They are embedded in Ruby by borrowing the syntax but their embedding is outward; their execution engines are their own.

For details, the documentation is available from Wiki.

Computation offloading from Ruby is a typical example of the DSLs implemented by Yadriggy. For example, Yadriggy provides a simple DSL to offload from Ruby to native C language.

require 'yadriggy/c'

include Yadriggy::C::CType

def fib(n) ! Integer
  typedecl n: Integer
  if n > 1
    return fib(n - 1) + fib(n - 2)
  else
    return n
  end
end

puts Yadriggy::C.run { return fib(32) }

When this code is run, the block given to Yadriggy::C.run is translated into C code with the definition of fib method. Then the C code is compiled into a dynamic library, loaded the library through ruby-ffi, and executed. Since the block given to run calls fib, the definition of fib is also translated into C.

An external variable is accessible from the compiled block:

n = 32
puts Yadriggy::C.run { return fib(n) }

The argument to fib is take from the variable n that exists outside of the block. n is passed to the compiled block by copying. It is not passed by reference. Thus, when a new value is assigned to n within the compiled block, it is not visible from the Ruby code. The variable n in the Ruby code keeps the old value.

Note that the definition of fib contains type declarations since this DSL is not Ruby. This DSL looks like Ruby but it is a different language. ! Integer following def fib(n) specifies the return type. typedecl specifies the types of the parameters (and local variables if any). In this DSL, most types have to be statically given although the DSL performs simple type inference and some types can be omitted.

Yadriggy provides a method for obtaining the abstract syntax tree (AST) of the given method, lambda, or Proc. It also provides a syntax checker that determines whether or not an AST satisfies the syntax described in the BNF-like language, which is a DSL embedded in Ruby by Yadriggy.

You can even obtain the AST of a piece of source code:

require 'yadriggy'
ast = Yadriggy.reify {|a| a + 1 }

reify returns the AST of the given block {|a| a + 1 }. It takes not only a block but also a Method or Proc object.

Yadriggy works with Pry and IRuby unless a syntax error occurs.

The idea of reify was proposed in the following paper:

• Shigeru Chiba, YungYu Zhuang, Maximilian Scherr, "Deeply Reifying Running Code for Constructing a Domain-Specific Language", PPPJ'16, Article No. 1, ACM, August 2016.

Yadriggy-Py was presented in the following paper:

• Shigeru Chiba, Foreign language interfaces by code migration, Proc. of the 18th ACM SIGPLAN International Conference on Generative Programming: Concepts and Experiences (GPCE 2019), pp. 1-13, ACM, 2019.

To install, run:

$ gem install yadriggy

or, download this repository and run:

$ bundle exec rake install

Bug reports and pull requests are welcome on GitHub at https://github.com/csg-tokyo/yadriggy.

The gem is available as open source under the terms of the MIT License.