Swarley's crazy type-checking method generator!

swarley_method_type.rb

class Class
  def define(method_name, argument_types = {}, &block)
    raise ArgumentError, "Block required" unless block_given?
    raise ArgumentError, "Expected argument Hash" unless argument_types.is_a? Hash
    raise ArgumentError, "Expected argument names to be Symbols" unless argument_types.each_key.all? {|x| x.is_a? Symbol }
    raise ArgumentError, "Expected argument types to be Classes" unless argument_types.each_value.all? {|x| x.is_a? Class }
    
    wrapper_class = Struct.new *argument_types.keys
    
    # Redirect all other methods to the class, so methods get called there not on the bearer of arguments.
    wrapper_class.send :define_method, :method_missing do |*args, &block|
      @_target.send *args, &block
    end
    
    define_method method_name do |*args|
      raise ArgumentError "Expected #{argument_types.size} arguments" unless args.size == argument_types.size
      
      argument_types.zip(args) do |(name, type), value|
        raise TypeError, "Expected argument #{name} to be a #{type.name} in #{method_name}" unless value.is_a? type
      end
      
      # Create a new wrapper with methods which act as fake arguments within the block.      
      wrapper = wrapper_class.new(*args)
      wrapper.instance_variable_set :@_target, self # So it can redirect to calling object.     
      wrapper.instance_eval &block
    end 
  end
end

class Bleh
  define :factorial, i: Integer do
    i < 1 ? 1 : i * factorial(i - 1)
  end
end

puts Bleh.new.factorial 5
# => 120

puts Bleh.new.factorial 5.4

=begin
swarley_method_type.rb:19:in `block (2 levels) in define': Expected argument i to be a Integer in factorial (TypeError)
        from swarley_method_type.rb:18:in `each'
        from swarley_method_type.rb:18:in `zip'
        from swarley_method_type.rb:18:in `block in define'
        from swarley_method_type.rb:39:in `<main>'
=end