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