Proposal? foldAlt :: (Foldable t, Alternative f) => t a -> f a

[Tony Morris]

I remember discussing this function about 8 or so years ago, in that it
generalises by composition.

Here it is being discussed in Scala in 2012, including the
generalisation by using the composition of functors. Sorry I couldn't
find the earlier Haskell discussions. They are out there somewhere.
https://groups.google.com/forum/#!msg/scala-user/O_udbztHFHU/mpqPtEkjbb8J

However, since the adoption of the lens library since, I have preferred
generalising based on Cons/Empty
https://github.com/qfpl/papa/blob/master/papa-lens-implement/src/Papa/Lens/Implement/Data/Collapse.hs


On 29/11/17 14:12, Fumiaki Kinoshita wrote:
> We found this quite useful, but we are not 100% about the name and
> documentation.
>
> -- | Try—in the 'Alternative' sense—to return all the values in a
> 'Foldable'
> -- container.
> --
> -- @
> -- foldAlt ≡ 'listToMaybe' :: [a] -> 'Maybe' a
> -- foldAlt ≡ 'maybeToList' :: 'Maybe' a -> [a]
> -- foldAlt ≡ 'MaybeT' . 'return' :: ('Monad' m) => 'Maybe' a ->
> 'MaybeT' m a
> -- foldAlt ≡ 'Pipes.ListT' . 'Pipes.each' :: ('Monad' m) => [a] ->
> 'Pipes.ListT' m a
> -- foldAlt ≡ id :: 'Maybe' a -> 'Maybe' a
> -- @
> foldAlt :: (Foldable t, Alternative f) => t a -> f a
> foldAlt = getAlt . foldMap (Alt . pure)
> {-# INLINE foldAlt #-}
>
> I propose adding this to either `Data.Foldable` or
> `Control.Applicative`. Any thoughts?
>
>
> _______________________________________________
> Libraries mailing list
> Libraries at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/libraries/attachments/20171129/86752b9a/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 488 bytes
Desc: OpenPGP digital signature
URL: <http://mail.haskell.org/pipermail/libraries/attachments/20171129/86752b9a/attachment.sig>