add Traversable generalizations of mapAccumL and mapAccumR
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.
Trac metadata
Trac field | Value |
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Version | |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | libraries/base |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | Unknown |
Architecture | Unknown |