Ala banisterfiend's cleverness: https://gist.github.com/banister/acec9b4688cbb7575106 - I made it do something else interesting

static_typed.rb

def static_typed(types={})
  type, method_name = types.first
  old_method = instance_method(method_name)
  define_method(method_name) do |*args, &block|
    old_method.bind(self).call(*args, &block).tap { |return_value|
      raise TypeError, "Return value is not of type #{type}!" unless type === return_value
    }
  end
end

# ---------------------------------

[8] pry(#<Foo>)> class Foo
[8] pry(#<Foo>)*   static_typed Integer => def foo(i) i end  
[8] pry(#<Foo>)* end  
=> :foo
[9] pry(#<Foo>)> Foo.new.foo(1)
=> 1
[10] pry(#<Foo>)> Foo.new.foo('s')
TypeError: Return value is not of type Integer!
from (pry):157:in `block (2 levels) in static_typed`

# Though you can be even more sneaky:
[14] pry(#<Foo>)> class Bar
[14] pry(#<Foo>)*   static_typed /[A-Z]+/ => def baz(s) s end
[14] pry(#<Foo>)* end  
=> :baz
[15] pry(#<Foo>)> Bar.new.baz('s')
TypeError: Return value is not of type (?-mix:[A-Z]+)!
from (pry):174:in `block (2 levels) in static_typed`
[16] pry(#<Foo>)> Bar.new.baz('S')
=> "S"