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

[Fumiaki Kinoshita]

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?

[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

Post by Fumiaki Kinoshita
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?
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[Andrew Martin]

+1 from me. We probably also want foldMapAlt:

foldMapAlt :: (Foldable t, Alternative f) => (a -> f b) -> t a -> f b

Post by Fumiaki Kinoshita
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?
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http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

[David Feuer]

I neither support nor oppose this in principle, but I definitely would
only support
it with a more clever implementation. The one above relies on arity analysis or
inlining to avoid building a lousy closure representing (Alt . pure).
foldMapAlt is really
nothing more than foldMap applied to a particular Monoid dictionary:

foldMapAlt :: forall t f a b. (Foldable t, Alternative f) => (a -> f
b) -> t a -> f b
foldMapAlt = coerce (foldMap :: (a -> Alt f b) -> t a -> Alt f b)

foldAlt :: (Foldable t, Alternative f) => t a -> f a
foldAlt = foldMapAlt pure

David Feuer

On Wed, Nov 29, 2017 at 7:01 AM, Andrew Martin

Post by Andrew Martin
foldMapAlt :: (Foldable t, Alternative f) => (a -> f b) -> t a -> f b

Post by Fumiaki Kinoshita
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?
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--
-Andrew Thaddeus Martin
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