Data.Traversable.Compat

Documentation

module Data.Traversable

mapAccumM :: (Monad m, Traversable t) => (s -> a -> m (s, b)) -> s -> t a -> m (s, t b)

The mapAccumM function behaves like a combination of mapM and mapAccumL that traverses the structure while evaluating the actions and passing an accumulating parameter from left to right. It returns a final value of this accumulator together with the new structure. The accummulator is often used for caching the intermediate results of a computation.

Examples

Basic usage:

>>> let expensiveDouble a = putStrLn ("Doubling " <> show a) >> pure (2 * a)
>>> :{
mapAccumM (\cache a -> case lookup a cache of
    Nothing -> expensiveDouble a >>= \double -> pure ((a, double):cache, double)
    Just double -> pure (cache, double)
    ) [] [1, 2, 3, 1, 2, 3]
:}
Doubling 1
Doubling 2
Doubling 3
([(3,6),(2,4),(1,2)],[2,4,6,2,4,6])

Since: base-4.18.0.0

forAccumM :: (Monad m, Traversable t) => s -> t a -> (s -> a -> m (s, b)) -> m (s, t b)

forAccumM is mapAccumM with the arguments rearranged.

Since: base-4.18.0.0