Runtime type checking in Ruby - proof-of-concept

types.rb

module Types
  InvalidTypeError = Class.new(ArgumentError)

  def specify(spec, a_method)
    args_spec = []
    return_spec = nil
    case spec.first
    when Hash
      args_spec = spec.first.keys.first
      return_spec = spec.first.values.first
    when Array
      args_spec = spec.first
    else
      args_spec = spec
    end

    original_method = :"_#{a_method}_without_type_checking"
    alias_method original_method, a_method
    define_method(a_method) do |*args|
      ensure_correct_types(args_spec, args)
      result = send(original_method, *args)
      ensure_correct_types(return_spec, [result], what: "return value") if return_spec
      result
    end

    a_method
  end

  module InstanceMethods
    def ensure_correct_types(spec, args, what: "argument")
      Array(spec).each.with_index do |klass, index|
        raise InvalidTypeError, ["Type for #{what}:", klass, "for" , args[index]&.inspect || "-missing #{index.succ} argument-", args[index].class].join(" ") if klass && !(klass === args[index])
      end
    end
  end
  private_constant :InstanceMethods


  def self.extended(mod)
    mod.include InstanceMethods
  end
end

Object.extend Types

class Foo
  specify [String],
  def foo(arg)
    42
  end

  specify [String => Numeric],
  def bar(arg)
    42
  end

  specify [[String, String]],
  def foo2(arg1, arg2)
    42
  end

  specify [[String, String] => Numeric],
  def bar2(arg1, arg2)
    42
  end
end


Foo.new.foo("42")
Foo.new.bar2("42", ?x)