[suzuki]

I have written almost the identical Scheme interpreters in Ruby and Crystal: [1] and [2]. The biggest difference I have felt between them is the absence of good old Object, which can represent everything at runtime, from Crystal. I had to declare Obj and Val:

  class Obj
  end

  # Value operated by Scheme
  alias Val = Nil | Obj | Bool | String | Int32 | Float64 | BigInt

to define Cons Cell of Scheme:

  # Cons cell
  class Cell < Obj
    include Enumerable(Val)

    getter car : Val            # Head part of the cell
    property cdr : Val          # Tail part of the cell

    def initialize(@car : Val, @cdr : Val)
    end

    ...
  end # Cell

Note that you see generics, Enumerable(Val), and constructor arguments with '@' in the excerpt above.

As for performance, Crystal is faster than Ruby 8.6 times as interpreter and 39.4 times as compiler [3]. You can use Crystal as a superfast (and typed) Ruby interpreter, in a sense.

[1] https://github.com/nukata/little-scheme-in-ruby [2] https://github.com/nukata/little-scheme-in-crystal [3] https://github.com/nukata/little-scheme#performance