[core libraries] Proposal: Make default impls for foldl1 and foldr1 lazier

[Edward Kmett]

+1 from me.

On Wed, Oct 29, 2014 at 3:08 PM, David Feuer <david.feuer at gmail.com> wrote:

> We currently have (in Data.Foldable)
>
>     foldr1 :: (a -> a -> a) -> t a -> a
>     foldr1 f xs = fromMaybe (error "foldr1: empty structure")
>                     (foldr mf Nothing xs)
>       where
>         mf x Nothing = Just x
>         mf x (Just y) = Just (f x y)
>
> and something similar for foldl1. This is strict in the entire spine,
> unlike the list version, because it has to get all the way to the end of
> the list before it starts laying down Justs. I propose we change this to
> the obvious:
>
>     foldr1 :: (a -> a -> a) -> t a -> a
>     foldr1 f xs = fromMaybe (error "foldr1: empty structure")
>                     (foldr mf Nothing xs)
>       where
>         mf x r = Just $ case r of
>                           Nothing -> x
>                           Just y  -> f x y
>
> Since GHC 7.10.1 is fast approaching, I doubt we have the usual two weeks
> to discuss this, so please speak up as soon as you can if you have concerns.
>
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