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/I... 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?
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