Proposal: Add Foldable helper to Control.DeepSeq

[David Feuer]

I've put up a pull request at https://github.com/haskell/deepseq/pull/18
adding rnfFoldableLTR and rnfFoldableRTL, and documenting when they can
(and when they cannot) produce good NFData instances. The implementations
are monoid based, but use slightly different monoid definitions than
originally proposed so as to produce extremely clean Core for GHC >= 7.8
even without specialization. Edward: similar functions could also be
written for Bifoldable. I'd be happy to send a PR to bifunctors if you'd
like.

On Jul 14, 2016 3:09 PM, "Edward Kmett" <ekmett at gmail.com> wrote:

> The monoid may be more efficient if the Foldable is constructed to take
> advantage of internal sharing.
>
> -Edward
>
> On Thu, Jul 14, 2016 at 11:52 AM, David Feuer <david.feuer at gmail.com>
> wrote:
>
>> 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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>
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