#2461: add Traversable generalizations of mapAccumL and mapAccumR
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Reporter: ross | Owner:
Type: proposal | Status: new
Priority: normal | Component: libraries/base
Version: | Severity: normal
Keywords: | Testcase:
Architecture: Unknown | Os: Unknown
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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.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2461>
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