Control.Monad.Sharing

Documentation

module Control.Monad

Classes

class Sharing m where

Interface of monads that support explicit sharing.

Methods

share :: Trans m a a => m a -> m (m a)

Yields an action that returns the same results as the given | action but whose effects are only executed once. Especially, | when the resulting action is duplicated it returns the same | result at every occurrence.

Instances

Monad m => Sharing (Lazy m)

class Trans m a b where

Interface to transform nested monadic data types. The provided | function trans is supposed to map the given function on every | monadic argument. The result of trans may be of the same type | as the argument but can also be of a different type, e.g. to | convert a value with nested monadic arguments to a corresponding | value without.

Methods

trans :: (forall c d. Trans m c d => m c -> m (m d)) -> a -> m b

Instances

Monad m => Trans m Double Double

Monad m => Trans m Float Float

Monad m => Trans m Char Char

Monad m => Trans m Int Int

Monad m => Trans m Bool Bool

(Monad m, Trans m a a) => Trans m [m a] [a]

An instance for lists with monadic elements that lifts all | monadic effects to the top level and yields a list with | non-monadic elements.

(Monad m, Trans m a a) => Trans m [m a] [m a]

An instance for lists with monadic elements.

(Monad m, Trans m a b) => Trans m (List m a) [b]

(Monad m, Trans m a b) => Trans m (List m a) (List m b)

We provide instances of the Trans class for some predefined Haskell types. For flat types the function trans just returns its argument which has no arguments to which the given function could be applied.

Evaluation

eval :: (Monad m, Trans m a b) => a -> m b

Lifts all monadic effects in nested monadic values to the top | level. If m is a monad for non-determinism and the argument a | data structure with nested non-determinism then the result | corresponds to the normal form of the argument.

Monadic lists

data List m a

Data type for lists where both the head and tail are monadic.

Constructors

Nil

Cons (m a) (m (List m a))

Instances

(Monad m, Trans m a b) => Trans m (List m a) [b]

(Monad m, Trans m a b) => Trans m (List m a) (List m b)

nil :: Monad m => m (List m a)

The empty monadic list.

cons :: Monad m => m a -> m (List m a) -> m (List m a)

Constructs a non-empty monadic list.

isEmpty :: Monad m => m (List m a) -> m Bool

Checks if monadic list is empty.

first :: MonadPlus m => m (List m a) -> m a

Yields the head of a monadic list. Relies on MonadPlus instance | to provide a failing implementation of fail.

rest :: MonadPlus m => m (List m a) -> m (List m a)

Yields the tail of a monadic list. Relies on MonadPlus instance | to provide a failing implementation of fail.