[Haskell - QuickCheck] Problem i Haskell.

readExpr :: String -> Maybe Expr
readExpr s = 
  case expr modS of
   Just (a,_) -> Just a
   _          -> Nothing
  where
    modS = filter (/= ' ') s

module ExprQC where

import Test.QuickCheck

import Expr

prop_ShowReadExpr :: Expr -> Bool
prop_ShowReadExpr expr =
  readExpr (show expr) == Just (assoc expr)

assoc :: Expr -> Expr
assoc (Add (Add a b) c) = assoc (Add a (Add b c))
assoc (Add a b)         = Add (assoc a) (assoc b)
assoc (Mul (Mul a b) c) = assoc (Mul a (Mul b c))
assoc (Mul a b)         = Mul (assoc a) (assoc b)
assoc (Sin a)           = Sin (assoc a)
assoc (Cos a)           = Cos (assoc a)
assoc a                 = a

GHCi, version 7.10.2: http://www.haskell.org/ghc/  :? for help
[1 of 2] Compiling Expr             ( Expr.hs, interpreted )
[2 of 2] Compiling ExprQC           ( ExprQC.hs, interpreted )
Ok, modules loaded: ExprQC, Expr.
*ExprQC> quickCheck prop_ShowReadExpr 
*** Failed! Falsifiable (after 1 test):  
Num 0.0