assert_types.rb

class Array
  def extract_options!
    last.is_a?(::Hash) ? pop : {}
  end
end

def assert_types *args, &block
  options = args.extract_options! 
  # Check that the block has the proper arity
  raise ArgumentError, "Expected #{options[:passed].count} arguments, got #{args.count}" unless options[:passed].count == args.count 

  # Check that the parameters have the proper "type"
  raise ArgumentError, "Method has signature (#{options[:passed].join(", ")}) => #{options[:returned]}" unless args.zip(options[:passed]).all? { |param, klass| param.is_a? klass } 
  out = block.call args

  # Check that the returned value conforms to the expected signature of the method
  raise TypeError, "Method has signature (#{options[:passed].join(", ")}) => #{options[:returned]}" unless out.is_a? options[:returned]
  out
end

def multiply x, y
  assert_types(x, y, {:passed => [Fixnum, Fixnum], :returned => Fixnum}) do |a,b|
    (a*b).to_s
  end
end

puts multiply "a", 5

# with_types.rb:14:in `assert_types': Method has signature (Fixnum, Fixnum) => Fixnum (ArgumentError)
#	from with_types.rb:23:in `multiply'
#	from with_types.rb:28