Hi guys I am a bit new to haskell but I am doing good till now. I have to write a function that takes 2 inputs and then reutns one composite output. Now my problem is that I have to make composition of that function meaning that I have to access in some way the output of the function before it is really computed. I will show you part of my code which is working prefectly: play :: Logo -> TurtleState -> (Image, TurtleState) play DoNothing (pen, (x,y), angle) = (emptyImage, (pen, (x,y), angle)) play PenDown (pen, (x,y), angle) = (emptyImage,(pen, (x,y), angle)) play PenUp (pen, (x,y), angle) = (emptyImage,(pen, (x,y), angle)) play (Forward n) (pen, (x,y), angle) |pen == True = ((line (x,y) (x+n,y+n)), (True, (x+n,y+n), angle)) |otherwise = (emptyImage,(pen, (x+n,y+n), angle)) play (Turn n) (pen, (x,y), angle) = (emptyImage, (pen, (x,y), (angle+n))) play (DoNothing :>: p2) (pen, (x,y), angle) = play p2 (pen, (x,y), angle) play (p1 :> DoNothing) (pen, (x,y), angle) = play p1 (pen, (x,y), angle) play (PenDown :>: PenUp) (pen, (x,y), angle) = (emptyImage,(pen, (x,y), angle)) play (PenDown :>: (Forward n)) (pen, (x,y), angle) = play (Forward n) (True, (x,y), angle) play (PenDown :>: PenDown) (pen, (x,y), angle) = (emptyImage,(pen, (x,y), angle)) play (PenDown :>: (Turn n)) (pen, (x,y), angle) = play (Turn n) (pen, (x,y), angle) play (PenUp :>: PenUp) (pen, (x,y), angle) = (emptyImage,(pen, (x,y), angle)) play (PenUp :>: (Forward n)) (pen, (x,y), angle) = play (Forward n) (False, (x,y), angle) play (PenUp :>: (Turn n)) (pen, (x,y), angle) = play (Turn n) (pen, (x,y), angle) play (PenUp :>: PenDown) (pen, (x,y), angle) = (emptyImage,(pen, (x,y), angle)) Now the problem comes here: play (p1 :>: p2) state |play p1 state == (i1,state1) && play p2 state1 == (i2,state2) = (i1+++i2,state2) I know that if I manage to do that function the one above with this sign :>: do not need to be impelmented since this one will cater for all the cases. Can you please help me? Thanks in advance! -- View this message in context: http://www.nabble.com/Small-question-about-something-easy-tp16119618p1611961... Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.