Adding foldMap implementation to instance Foldable Maybe

[David Feuer]

This does not require a proposal--it merely replaces an inefficient
implementation with an efficient one. If you're set up with Phabricator,
just submit a patch. Otherwise, I'll be happy to do it for you.

On Jan 19, 2017 12:22 AM, "Eric Mertens" <emertens at gmail.com> wrote:

> Hello,
>
> Independently of a blog post I just noticed tonight on reddit,
> http://www.snoyman.com/blog/2017/01/follow-up-mapm I observed a stack
> overflow in some code I was working with due to a recursive call inside the
> function argument of for_
>
> The code was a more complicated version of:
>
> go = for_ someMaybe $ \m -> m >> go
>
> It was wrong in thinking that this could be efficient because there’s an
> extra operation to set the result to a (). I thought I might try working
> around this with the following code.
>
> for__ :: (Applicative f, Foldable t) => t a -> (a -> f ()) -> f ()
> for__ xs f = unApp (foldMap (coerce f) xs)
>
> newtype App f = App { unApp :: f () }
>
> instance Applicative f => Monoid (App f) where
>   mempty                  = App (pure ())
>   mappend (App x) (App y) = App (x *> y)
>
> Unfortunately the instance for Foldable for Maybe does not provide an
> implementation of foldMap, it only provides foldl and foldr. This means
> that the derived foldMap implementation attempts to mappend an extra mempty
> because the default implementation of foldMap is
>
>  foldMap <http://hackage.haskell.org/package/base-4.9.1.0/docs/src/Data.Foldable.html#foldMap> f <http://hackage.haskell.org/package/base-4.9.1.0/docs/src/Data.Foldable.html#local-6989586621679114983> = foldr <http://hackage.haskell.org/package/base-4.9.1.0/docs/src/Data.Foldable.html#foldr> (mappend <http://hackage.haskell.org/package/base-4.9.1.0/docs/src/GHC.Base.html#mappend> . <http://hackage.haskell.org/package/base-4.9.1.0/docs/src/GHC.Base.html#.> f <http://hackage.haskell.org/package/base-4.9.1.0/docs/src/Data.Foldable.html#local-6989586621679114983>) mempty <http://hackage.haskell.org/package/base-4.9.1.0/docs/src/GHC.Base.html#mempty>
>
>
> With an explicit implementation of foldMap for the Maybe type (and maybe
> there are others that we’ve missed, for__ above can be stack efficient.
>
> So I propose that we add an explicit implementation of foldMap to the
> Foldable Maybe instance
>
>   foldMap _ Nothing  = mempty
>   foldMap f (Just x) = f x
>
>
> Best regards,
> Eric Mertens
>
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