streaming-nonempty

A correct groupBy function is expected to produce only non-empty groups, which means we should be able to fold them up with only Semigroup constraint on the values. This is not the case for Data.List.groupBy as well for the Streaming lib counterpart.

This package export a groupBy function that produce f (Stream f m a) groups which are then guaranteed to have the functorial layer.

The NEStream (Of a) m r newtype is supporting sconcat which means we can define

groupSemigroupBy :: (Semigroup a, Monad m) => (a -> a -> Bool) -> Stream (Of a) m r -> Stream (Of a) m r
groupSemigroupBy f = S.mapped sconcat . groupBy f

with expected behavior to collapse groups using semigroup composition

In contrast using the standard groupBy we are stuck with

groupMonoidBy :: (Monoid a, Monad m) => (a -> a -> Bool) -> Stream (Of a) m r -> Stream (Of a) m r
groupMonoidBy f = S.mapped mconcat . groupBy f

It would be legit to use an sconcatUnsafe that would panic on empty streams because we know groups are not empty.