Add mapWithState

[BalmungSan]

The signature would be something like:

def mapWithState[S, B](init: S)(f: (S, A) => (S, B)): Col[B]

Is not as common as mapWithIndex but is kind of handy and is very hard to implement generically without much boilerplate.

[benhutchison]

Id do this with a fold normally..And IMO the final state value needs to be exposed..

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[BalmungSan]

Id do this with a fold normally

The thing is that now you have to construct the new collection by yourself which is redundant and error-prone but also it may be more inefficient than the underlying map

And IMO the final state value needs to be exposed

Yeah, that is a good idea.

PS: For the record, fs2 has this function, is called mapAccumulate
And I am creating a PR to add it to cats.

[He-Pin]

mapAccululate or mapAccumulation or statefulMap

[sageserpent-open]

Just mentioned this here: https://users.scala-lang.org/t/idiomatic-map-combined-with-fold/9628, which lead me here. Had the need for this on several occasions.