Proposal: Add Foldable helper to Control.DeepSeq

[David Feuer]

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" <david.feuer at gmail.com> wrote:

> 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` ()
>
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