The prelude support for strict evaluation, `seq` and ($!), evaluate only enough to ensure that the value being "forced" is not bottom. In your case you need a "deeper" evaluation to be forced. A clean (though somewhat tedious) way to achieve what you need is with the `deepSeq` function from the following module. Dean Herington The `DeepSeq` class provides a method `deepSeq` that is similar to `seq` except that it forces deep evaluation of its first argument before returning its second argument. Instances of `DeepSeq` are provided for Prelude types. Other instances must be supplied by users of this module.
module DeepSeq where
class DeepSeq a where deepSeq :: a -> b -> b deepSeq = seq -- default, for simple cases
infixr 0 `deepSeq`, $!!
($!!) :: (DeepSeq a) => (a -> b) -> a -> b f $!! x = x `deepSeq` f x
instance DeepSeq () where
instance (DeepSeq a) => DeepSeq [a] where deepSeq [] y = y deepSeq (x:xs) y = deepSeq x $ deepSeq xs y
instance (DeepSeq a,DeepSeq b) => DeepSeq (a,b) where deepSeq (a,b) y = deepSeq a $ deepSeq b y instance (DeepSeq a,DeepSeq b,DeepSeq c) => DeepSeq (a,b,c) where deepSeq (a,b,c) y = deepSeq a $ deepSeq b $ deepSeq c y instance (DeepSeq a,DeepSeq b,DeepSeq c,DeepSeq d) => DeepSeq (a,b,c,d) where deepSeq (a,b,c,d) y = deepSeq a $ deepSeq b $ deepSeq c $ deepSeq d y instance (DeepSeq a,DeepSeq b,DeepSeq c,DeepSeq d,DeepSeq e) => DeepSeq (a,b,c,d,e) where deepSeq (a,b,c,d,e) y = deepSeq a $ deepSeq b $ deepSeq c $ deepSeq d $ deepSeq e y instance (DeepSeq a,DeepSeq b,DeepSeq c,DeepSeq d,DeepSeq e,DeepSeq f) => DeepSeq (a,b,c,d,e,f) where deepSeq (a,b,c,d,e,f) y = deepSeq a $ deepSeq b $ deepSeq c $ deepSeq d $ deepSeq e $ deepSeq f y instance (DeepSeq a,DeepSeq b,DeepSeq c,DeepSeq d,DeepSeq e,DeepSeq f,DeepSeq g) => DeepSeq (a,b,c,d,e,f,g) where deepSeq (a,b,c,d,e,f,g) y = deepSeq a $ deepSeq b $ deepSeq c $ deepSeq d $ deepSeq e $ deepSeq f $ deepSeq g y
instance DeepSeq Bool where instance DeepSeq Char where
instance (DeepSeq a) => DeepSeq (Maybe a) where deepSeq Nothing y = y deepSeq (Just x) y = deepSeq x y
instance (DeepSeq a, DeepSeq b) => DeepSeq (Either a b) where deepSeq (Left a) y = deepSeq a y deepSeq (Right b) y = deepSeq b y
instance DeepSeq Ordering where
instance DeepSeq Integer where instance DeepSeq Int where instance DeepSeq Float where instance DeepSeq Double where