mapAccumL and variants

[andrewthad]

In Data.List, there is a function that maps over a collection while building an accumulator:

mapAccumL :: Traversable t => (a -> b -> (a, c)) -> a -> t b -> (a, t c) 

Specialized to Array, this would be:

mapAccumL :: (a -> b -> (a, c)) -> a -> Array b -> (a, Array c)

This is basically a combination of foldl and map. There is a less expressive variant of this where the accumulator is a Monoid and you do not inspect it (a comparison to the contrast between Writer and State is apt here). This variant looks like this:

mapAccumMonoid :: Monoid a => (b -> (a, c)) -> Array b -> (a, Array c)

I use this function somewhat often. I've thought about adding it to primitive, but I'm going to just add it to contiguous for the time being. Just wanted to get these thoughts written down somewhere.