gistfile1.txt

# When anything, including nil, is a valid param and you need
# a clear way to know if a param was set or not.

def foo a, b = (b_was_not_passed = true; nil)
  result = [a]
  result << b unless b_was_not_passed
  result
end

DEVIL.

Wow. Just wow.

Similar idea for when you receive an argument or a block:

def on(error, message = (block_given? ? yield : raise(ArgumentError)))

From https://github.com/soveran/ohm/blob/master/lib/ohm/validations.rb#L77-81

if you dont care about the exception class, you can also do a def on(error, message = yield).