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 [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries |
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. However, since the adoption of the lens library since, I have
preferred generalising based on Cons/Empty On 29/11/17 14:12, Fumiaki Kinoshita
wrote:
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+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 <[hidden email]> wrote:
-Andrew Thaddeus Martin
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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 <[hidden email]> 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 <[hidden email]> > 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 >> [hidden email] >> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries >> > > > > -- > -Andrew Thaddeus Martin > > _______________________________________________ > Libraries mailing list > [hidden email] > http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries > Libraries mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries |
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