Control.Monad.Plus

Partial maps and filters over MonadPlus instances.

This is especially useful for sequential structures such as event lists, tracks etc.

Inspired by the Conal Elliott's blog post:

• http://conal.net/blog/posts/a-handy-generalized-filter

Basics

module Control.Monad

msum :: MonadPlus m => [m a] -> m a

This generalizes the list-based concat function.

msum' :: (MonadPlus m, Foldable t) => t (m a) -> m a

This generalizes the list-based concat function.

Constructing

mfold :: (MonadPlus m, Foldable t) => t a -> m a

Fold a value into an arbitrary MonadPlus type.

This function generalizes the toList function.

mfromList :: MonadPlus m => [a] -> m a

Translate a list to an arbitrary MonadPlus type.

This function generalizes the listToMaybe function.

mfromMaybe :: MonadPlus m => Maybe a -> m a

Translate maybe to an arbitrary MonadPlus type.

This function generalizes the maybeToList function.

Filtering

mfilter' :: MonadPlus m => (a -> Bool) -> m a -> m a

mfilter', applied to a predicate and a container, returns the container of those elements that satisfy the predicate; i.e.,

 filter p xs = [ x | x <- xs, p x]

This function generalizes the filter function. (Identical to mfilter, it is just here for documentation purposes).

mpartition :: MonadPlus m => (a -> Bool) -> m a -> (m a, m a)

The partition function takes a predicate a list and returns the pair of lists of elements which do and do not satisfy the predicate, respectively; i.e.,

 partition p xs == (filter p xs, filter (not . p) xs)

This function generalizes the partition function.

Special filters

mscatter :: MonadPlus m => m [b] -> m b

Pass through Just occurrences.

This function generalizes the catMaybes function.

mscatter' :: (MonadPlus m, Foldable t) => m (t b) -> m b

Pass through Just occurrences.

This function generalizes the catMaybes function.

mcatMaybes :: MonadPlus m => m (Maybe a) -> m a

Pass through Just occurrences.

This function generalizes the catMaybes function.

mlefts :: MonadPlus m => m (Either a b) -> m a

Pass through Left occurrences.

This function generalizes the lefts function.

mrights :: MonadPlus m => m (Either a b) -> m b

Pass through Right occurrences.

This function generalizes the rights function.

mpartitionEithers :: MonadPlus m => m (Either a b) -> (m a, m b)

Separate Left and Right occurances.

This function generalizes the partitionEithers function.

Special maps

mmapMaybe :: MonadPlus m => (a -> Maybe b) -> m a -> m b

Modify or discard a value.

This function generalizes the mapMaybe function.

mconcatMap :: MonadPlus m => (a -> [b]) -> m a -> m b

Modify and return a number of values.

This function generalizes the concatMap function.