Safe Haskell	Safe-Inferred
Language	Haskell2010

Traq.Control.Monad

Contents

Traversable
MonadState
MonadWriter
MonadError
Helpers

Synopsis

mapAccumM :: (Monad m, Traversable t) => (s -> a -> m (s, b)) -> s -> t a -> m (s, t b)
forAccumM :: (Monad m, Traversable t) => s -> t a -> (s -> a -> m (s, b)) -> m (s, t b)
withSandboxOf :: MonadState s m => Lens' s s' -> m a -> m a
withSandbox :: MonadState s m => m a -> m a
tell :: MonadWriter w m => w -> m ()
tellAt :: MonadWriter w m => Lens' w w' -> w' -> m ()
writeElemAt :: (Applicative f, MonadWriter w m) => Lens' w (f a) -> a -> m ()
writeElem :: (Applicative f, MonadWriter (f a) m) => a -> m ()
ignoreWriter :: (Monad m, Monoid w') => RWST r w s m a -> RWST r w' s m a
throwFrom :: MonadError MyError m => m a -> MyError -> m a
singularM :: MonadError e m => Getting (Endo [a]) s a -> e -> s -> m a
maybeWithError :: MonadError e m => e -> Maybe a -> m a
liftEither :: MonadError e m => Either e a -> m a
(??) :: Functor f => f (a -> b) -> a -> f b
non' :: a -> SimpleGetter (Maybe a) a

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

Expand

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