ListT

Documentation

newtype ListT m a

A proper implementation of the list monad-transformer. Useful for streaming of monadic data structures.

Since it has instances of MonadPlus and Alternative, you can use general utilities packages like "monadplus" with it.

Constructors

ListT (m (Maybe (a, ListT m a)))

Instances

MFunctor ListT

MMonad ListT

MonadTrans ListT

MonadBase b m => MonadBase b (ListT m)

MonadBaseControl b m => MonadBaseControl b (ListT m)

MonadError e m => MonadError e (ListT m)

Monad m => Monad (ListT m)

Functor m => Functor (ListT m)

(Monad m, Functor m) => Applicative (ListT m)

(Monad m, Functor m) => Alternative (ListT m)

Monad m => MonadPlus (ListT m)

MonadIO m => MonadIO (ListT m)

Monad m => Monoid (ListT m a)

type StM (ListT m) a = StM m (Maybe (a, ListT m a))

Execution utilities

uncons :: ListT m a -> m (Maybe (a, ListT m a))

Execute in the inner monad, getting the head and the tail. Returns nothing if it's empty.

head :: Monad m => ListT m a -> m (Maybe a)

Execute, getting the head. Returns nothing if it's empty.

tail :: Monad m => ListT m a -> m (Maybe (ListT m a))

Execute, getting the tail. Returns nothing if it's empty.

null :: Monad m => ListT m a -> m Bool

Execute, checking whether it's empty.

fold :: Monad m => (r -> a -> m r) -> r -> ListT m a -> m r

Execute, applying a left fold.

foldMaybe :: Monad m => (r -> a -> m (Maybe r)) -> r -> ListT m a -> m r

A version of fold, which allows early termination.

toList :: Monad m => ListT m a -> m [a]

Execute, folding to a list.

toReverseList :: Monad m => ListT m a -> m [a]

Execute, folding to a list in the reverse order. Performs more efficiently than toList.

traverse_ :: Monad m => (a -> m ()) -> ListT m a -> m ()

Execute, traversing the stream with a side effect in the inner monad.

splitAt :: Monad m => Int -> ListT m a -> m ([a], ListT m a)

Execute, consuming a list of the specified length and returning the remainder stream.

Construction utilities

cons :: Monad m => a -> ListT m a -> ListT m a

Prepend an element.

fromFoldable :: (Monad m, Foldable f) => f a -> ListT m a

Construct from any foldable.

fromMVar :: MonadIO m => MVar (Maybe a) -> ListT m a

Construct from an MVar, interpreting the value of Nothing as the end.

unfold :: Monad m => (b -> Maybe (a, b)) -> b -> ListT m a

Construct by unfolding a pure data structure.

unfoldM :: Monad m => (b -> m (Maybe (a, b))) -> b -> ListT m a

Construct by unfolding a monadic data structure

This is the most memory-efficient way to construct ListT where the length depends on the inner monad.

repeat :: Monad m => a -> ListT m a

Produce an infinite stream.

Transformation utilities

These utilities only accumulate the transformations without actually traversing the stream. They only get applied in a single traversal, which only happens at the execution.

traverse :: Monad m => (a -> m b) -> ListT m a -> ListT m b

A transformation, which traverses the stream with an action in the inner monad.

take :: Monad m => Int -> ListT m a -> ListT m a

A transformation, reproducing the behaviour of Data.List.take.

drop :: Monad m => Int -> ListT m a -> ListT m a

A transformation, reproducing the behaviour of Data.List.drop.

slice :: Monad m => Int -> ListT m a -> ListT m [a]

A transformation, which slices a list into chunks of the specified length.