Missing combinators for creating and consuming Fold1, and IndexedFold1 's

[noinia]

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

[gnull]

Would you @ekmett accept a PR for this?

[ekmett]

Given we export Fold1, I think it'd be reasonable to supply basic combinators for working with them like this. So yes, I would accept a PR.

[noinia]

I've made a PR that implements the above functions. In a similar vain as this issue, I'm missing maximum1ByOf, minimum1ByOf analogues of maximumByOf and minimumByOf. If you are open to adding combinators like that as well, I can open another PR.

[ekmett]

With those additions, I'm wondering if we shouldn't exile these (along with the combinators in the PR above) to a Control.Lens.Fold1 or Control.Lens.Semigroup.Fold as they do require more "power" than that of a Fold, so they seem to be somewhat misplaced. Then we can decide if it makes sense to design that module for named import or regular, and whether these names should be in the default export set for lens.