I now have a vague clue how to do what I wanted. I wanted to substitute types in place... a silly example would be to convert Ints to Floats and Floats to Ints... (3 :: Int, 4.0 :: Float) -> (3.0 :: Float, 4 :: Int) I already have a function that will convert the input to: [("3",IntType),("4.0",FloatType)] So I can then manipulate the list in this form to: [("3.0",FloatType),("4",IntType)] All I now need to do is reconstruct back into the data structure... I have however been unable to find any docs on "gunfold" which I think is the way to do this... If somebody point me to some documentation for gunfold, or help in any way I would be very grateful... Regards, Keean Schupke.