Subject: proposal #2461: add Traversable generalizations of mapAccumL and mapAccumR
Newsgroups: gmane.comp.lang.haskell.libraries
Date: 2008-07-22 16:12:48 GMT (49 weeks, 5 days, 6 hours and 58 minutes ago)
The proposal is to add the following functions to Data.Traversable,
generalizing the list versions in Data.List:
-- |The 'mapAccumL' function behaves like a combination of 'fmap'
-- and 'foldl'; it applies a function to each element of a structure,
-- passing an accumulating parameter from left to right, and returning
-- a final value of this accumulator together with the new structure.
mapAccumL :: Traversable t => (a -> b -> (a, c)) -> a -> t b -> (a, t c)
-- |The 'mapAccumR' function behaves like a combination of 'fmap'
-- and 'foldr'; it applies a function to each element of a structure,
-- passing an accumulating parameter from right to left, and returning
-- a final value of this accumulator together with the new structure.
mapAccumR :: Traversable t => (a -> b -> (a, c)) -> a -> t b -> (a, t c)
These functions are handy for things like labelling trees, zipping, etc.
Deadline: 3 weeks