Assuming that z :: Int, you can declare an algebraic datatype data TwoOrThree a b = Three (a, b, Int) | Two (a, b) deriving(Show, Eq) -- so you can experiment And then define expand as expand :: TwoOrThree a b -> (a, b, Int) expand (Three tuple) = tuple expand (Two (a, b)) = (a, b, 1) Tom (amindfv) On Oct 2, 2011 6:04 AM, "Du Xi" wrote: