mapAccum package:ghc-lib-parser

The function mapAccum threads an accumulating argument through the map in ascending order of keys.

let f a b = (a ++ b, b ++ "X")
mapAccum f "Everything: " (fromList [(5,"a"), (3,"b")]) == ("Everything: ba", fromList [(3, "bX"), (5, "aX")])

The [CoAxBranch] passed into the mapping function is a list of all previous branches, reversed

Note this is not very efficient because it traverses the whole stream before rebuilding it, avoid using it if you can. mapAccumL used to implemented but it wasn't used anywhere in the compiler and has similar efficiency problems.

The function mapAccumRWithKey threads an accumulating argument through the map in descending order of keys.

The function mapAccumWithKey threads an accumulating argument through the map in ascending order of keys.

let f a k b = (a ++ " " ++ (show k) ++ "-" ++ b, b ++ "X")
mapAccumWithKey f "Everything:" (fromList [(5,"a"), (3,"b")]) == ("Everything: 3-b 5-a", fromList [(3, "bX"), (5, "aX")])

Monadic version of mapAccumL