I guess these are equivalent to the simpler expressions
rnfLTR = foldl' (const rnf) ()
rnfRTL = foldr' (\x _ -> rnf x) ()
On Jul 13, 2016 11:16 PM, "David Feuer"
As I describe in https://github.com/haskell/deepseq/issues/17 it is possible to implement an NFData instance for any Foldable type, and we can offer a function to help do that:
data Unit = Unit
instance Monoid Unit where mempty = Unit Unit `mappend` Unit = Unit -- strict in both arguments, unlike ()
rnfFoldable :: (Foldable f, NFData a) => f a -> () rnfFoldable xs = foldMap (\x -> rnf x `seq` Unit) xs `seq` ()
This could be used like so:
instance NFData a => NFData (F a) where rnf = rnfFoldable
This version forces from left to right. It would be possible to offer another version that forces from right to left:
data Unit2 = Unit2
instance Monoid Unit2 where mempty = Unit2 x `mappend` Unit2 = x `seq` Unit2
rnfFoldableRTL :: (Foldable f, NFData a) => f a -> () rnfFoldableRTL xs = foldMap (\x -> rnf x `seq` Unit2) xs `seq` ()