Control.Monad.List.Trans

newtype ListT f a

The list monad transformer.

This monad transformer extends the base monad with non-determinism. That is, the transformed monad supports the same effects as the base monad but with multiple return values.

Constructors

• ListT (f (Step a (ListT f a)))

Instances

• Newtype (ListT f a) _
• (Applicative f) => Semigroup (ListT f a)
• (Applicative f) => Monoid (ListT f a)
• (Functor f) => Functor (ListT f)
• (Monad f) => Unfoldable (ListT f)
• (Monad f) => Unfoldable1 (ListT f)
• (Monad f) => Apply (ListT f)
• (Monad f) => Applicative (ListT f)
• (Monad f) => Bind (ListT f)
• (Monad f) => Monad (ListT f)
• MonadTrans ListT
• (Applicative f) => Alt (ListT f)
• (Monad f) => Plus (ListT f)
• (Monad f) => Alternative (ListT f)
• (Monad f) => MonadZero (ListT f)
• (Monad f) => MonadPlus (ListT f)
• (MonadEffect m) => MonadEffect (ListT m)

data Step a s

The result of a single step in a ListT computation. Either:

• Computation has finished (Done), or
• A result has been returned, along with the next part of the computation (Yield).

The Skip constructor allows us to avoid traversing lists during certain operations.

Constructors

• Yield a (Lazy s)
• Skip (Lazy s)
• Done

catMaybes :: forall a f. Functor f => ListT f (Maybe a) -> ListT f a

Remove elements from a list which do not contain a value.

cons :: forall a f. Applicative f => Lazy a -> Lazy (ListT f a) -> ListT f a

Attach an element to the front of a list.

drop :: forall a f. Applicative f => Int -> ListT f a -> ListT f a

Drop a number of elements from the front of a list.

dropWhile :: forall a f. Applicative f => (a -> Boolean) -> ListT f a -> ListT f a

Drop elements from the front of a list while a predicate holds.

filter :: forall a f. Functor f => (a -> Boolean) -> ListT f a -> ListT f a

Remove elements from a list for which a predicate fails to hold.

foldl :: forall b a f. Monad f => (b -> a -> b) -> b -> ListT f a -> f b

Fold a list from the left, accumulating the result using the specified function.

foldlRec :: forall b a f. MonadRec f => (b -> a -> b) -> b -> ListT f a -> f b

Fold a list from the left, accumulating the result using the specified function. Uses tail call optimization.

foldl' :: forall b a f. Monad f => (b -> a -> f b) -> b -> ListT f a -> f b

Fold a list from the left, accumulating the result (effectfully) using the specified function.

foldlRec' :: forall b a f. MonadRec f => (b -> a -> f b) -> b -> ListT f a -> f b

Fold a list from the left, accumulating the result (effectfully) using the specified function. Uses tail call optimization.

fromEffect :: forall a f. Applicative f => f a -> ListT f a

Lift a computation from the base functor.

head :: forall a f. Monad f => ListT f a -> f (Maybe a)

Extract the first element of a list.

iterate :: forall a f. Monad f => (a -> a) -> a -> ListT f a

Generate an infinite list by iterating a function.

mapMaybe :: forall b a f. Functor f => (a -> Maybe b) -> ListT f a -> ListT f b

Apply a function to the elements of a list, keeping only those return values which contain a result.

nil :: forall a f. Applicative f => ListT f a

The empty list.

prepend :: forall a f. Applicative f => a -> ListT f a -> ListT f a

Prepend an element to a list.

prepend' :: forall a f. Applicative f => a -> Lazy (ListT f a) -> ListT f a

Prepend an element to a lazily-evaluated list.

repeat :: forall a f. Monad f => a -> ListT f a

Generate an infinite list by repeating a value.

runListT :: forall a f. Monad f => ListT f a -> f Unit

Drain a ListT, running it to completion and discarding all values.

runListTRec :: forall a f. MonadRec f => ListT f a -> f Unit

Drain a ListT, running it to completion and discarding all values. Stack safe: Uses tail call optimization.

scanl :: forall b a f. Monad f => (b -> a -> b) -> b -> ListT f a -> ListT f b

Fold a list from the left, accumulating the list of results using the specified function.

singleton :: forall a f. Applicative f => a -> ListT f a

Create a list with one element.

tail :: forall a f. Monad f => ListT f a -> f (Maybe (ListT f a))

Extract all but the first element of a list.

take :: forall a f. Applicative f => Int -> ListT f a -> ListT f a

Take a number of elements from the front of a list.

takeWhile :: forall a f. Applicative f => (a -> Boolean) -> ListT f a -> ListT f a

Take elements from the front of a list while a predicate holds.

uncons :: forall a f. Monad f => ListT f a -> f (Maybe (Tuple a (ListT f a)))

Perform the first step of a computation in the ListT monad.

unfold :: forall z a f. Monad f => (z -> f (Maybe (Tuple z a))) -> z -> ListT f a

Unfold a list using an effectful generator function.

wrapEffect :: forall a f. Functor f => f (ListT f a) -> ListT f a

Lift a computation from the base monad.

wrapLazy :: forall a f. Applicative f => Lazy (ListT f a) -> ListT f a

Defer evaluation of a list.

zipWith :: forall c b a f. Monad f => (a -> b -> c) -> ListT f a -> ListT f b -> ListT f c

Zip the elements of two lists, combining elements at the same position from each list.

zipWith' :: forall c b a f. Monad f => (a -> b -> f c) -> ListT f a -> ListT f b -> ListT f c

Zip the elements of two lists, combining elements at the same position from each list.

class MonadTrans t  where

The MonadTrans type class represents monad transformers.

A monad transformer is a type constructor of kind (* -> *) -> * -> *, which takes a Monad as its first argument, and returns another Monad.

This allows us to add additional effects to an existing monad. By iterating this process, we create monad transformer stacks, which contain all of the effects required for a particular computation.

The laws state that lift is a Monad morphism.

Laws:

• lift (pure a) = pure a
• lift (do { x <- m ; y }) = do { x <- lift m ; lift y }

Members

• lift :: forall a m. Monad m => m a -> t m a