type constraint

constraint.rb

class Module
  class ConstraintError < TypeError; end

  def where(meth, constraint)
    m = Module.new do
      define_method(meth) do |*args|
        args.zip(constraint.keys.first) do |arg, t|
          raise ConstraintError, "type #{t} is expected, but #{arg.inspect} is passed" unless arg.is_a?(t)
        end
        ret = super(*args)
        t = constraint.values.first
        raise ConstraintError, "type #{t} is expected, but #{ret.inspect} is returned" unless ret.is_a?(t)
        ret
      end
    end
    self.prepend m
  end
end

class Foo
  where def foo(n, s)
    puts "#{s}: #{n}"
  end, [Integer, String] => NilClass
end

f = Foo.new.foo(10, "A")

つまんないし名前が悪い。