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 wrote:

+1 from me. We probably also want foldMapAlt:

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

On Tue, Nov 28, 2017 at 11:12 PM, 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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