Enforcing Strict Evaluation
Dean Herington
heringto@cs.unc.edu
Fri, 17 Aug 2001 16:41:24 -0400 (EDT)
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