[Haskell-cafe] Small question about something easy

[iliali16]

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!
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