Draft implementation for Rec

x.rb

class DatatypeBase
  def initialize(blk)
    @blk = blk
  end

  def _resolve!
    @raw_contract = instance_eval(&@blk)
  end

  def Rec(name, &blk)
    RecExtension.Rec(name, self.class, &blk)
  end
end

module RecExtension
  def self.Rec(name, base=DatatypeBase, &blk)
    klass = Class.new(base)
    instance = klass.new(blk)

    klass.send(:define_method, name) do
      instance
    end

    instance._resolve!

    instance
  end

  def Rec(*args, &blk)
    RecExtension.Rec(*args, &blk)
  end
end

RecExtension.Rec(:a) { Rec(:b) { [ Integer, a => b ] } }
# => #<#<Class:0x0000000203b838>:0x0000000203b7c0 @blk=#<Proc:0x0000000203b888@(irb):3>, @raw_contract=#<#<Class:0x0000000203b630>:0x0000000203b5e0 @blk=#<Proc:0x0000000203b658@(irb):3>, @raw_contract=[Integer, {#<#<Class:0x0000000203b838>:0x0000000203b7c0 ...>=>#<#<Class:0x0000000203b630>:0x0000000203b5e0 ...>}]>>