Missing combinators for creating and consuming Fold1, and IndexedFold1 's #1015
Description
When working with Fold1 and/or IndexedFold1's it would be nice to have functions such as 'folding1' and 'ifolding1' to build the appropriate non-empty folds, and an indexed version of 'toNonEmptyOf' to consume them. In particular, I would suggest the following implementations:
-- | construct a Fold1 from a function that produces a Foldable1
folding1 :: Foldable1 f => (s -> f a) -> Fold1 s a
folding1 sfa agb = phantom . traverse1_ agb . sfa
{-# INLINE folding1 #-}
-- | Version of ifolding to build an 'IndexedFold1'
ifolding1 :: (Foldable1 f, Indexable i p, Contravariant g, Apply g)
=> (s -> f (i, a)) -> Over p g s t a b
ifolding1 sfa f = phantom . traverse1_ (phantom . uncurry (indexed f)) . sfa
{-# INLINE ifolding1 #-}
-- | indexed version of 'toNonEmptyOf'
itoNonEmptyOf :: IndexedGetting i (NonEmptyDList (i,a)) s a -> s -> NonEmpty (i,a)
itoNonEmptyOf l = flip getNonEmptyDList [] . ifoldMapOf l (\i a -> NonEmptyDList $ ((i,a) :|))
{-# INLINE itoNonEmptyOf #-}The implementations of folding1 and ifolding1 are almost identical to folding and ifolding (but with a different typeclass constraint). The implementation of iToNonEmptyOf was adapted from toNonEmptyOf