Module

Control.Monad.Writer.Trans

This module defines the writer monad transformer, WriterT.

#WriterT

newtype WriterT :: Type -> (Type -> Type) -> Type -> Typenewtype WriterT w m a

The writer monad transformer.

This monad transformer extends the base monad with a monoidal accumulator of type w.

The MonadWriter type class describes the operations supported by this monad.

Constructors

Instances

#runWriterT

runWriterT :: forall w m a. WriterT w m a -> m (Tuple a w)

Run a computation in the WriterT monad.

#execWriterT

execWriterT :: forall w m a. Functor m => WriterT w m a -> m w

Run a computation in the WriterT monad, discarding the result.

#mapWriterT

mapWriterT :: forall w1 w2 m1 m2 a b. (m1 (Tuple a w1) -> m2 (Tuple b w2)) -> WriterT w1 m1 a -> WriterT w2 m2 b

Change the accumulator and base monad types in a WriterT monad action.

Re-exports from Control.Monad.Trans.Class

#MonadTrans

class MonadTrans :: ((Type -> Type) -> Type -> Type) -> Constraintclass 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

Re-exports from Control.Monad.Writer.Class

#MonadTell

class MonadTell :: Type -> (Type -> Type) -> Constraintclass (Semigroup w, Monad m) <= MonadTell w m | m -> w where

The MonadTell w type class represents those monads which support a monoidal accumulator of type w, where tell appends a value to the accumulator.

An implementation is provided for WriterT, and for other monad transformers defined in this library.

Law:

  • do { tell x ; tell y } = tell (x <> y)

Members

#MonadWriter

class MonadWriter :: Type -> (Type -> Type) -> Constraintclass (Monoid w, MonadTell w m) <= MonadWriter w m | m -> w where

An extension of the MonadTell class that introduces some operations on the accumulator:

  • listen modifies the result to include the changes to the accumulator.
  • pass applies the returned function to the accumulator.

An implementation is provided for WriterT, and for other monad transformers defined in this library.

Laws in addition to the MonadTell law:

  • do { tell x ; tell y } = tell (x <> y)
  • listen (pure a) = pure (Tuple a mempty)
  • listen (writer a x) = tell x $> Tuple a x

Members

#listens

listens :: forall w m a b. MonadWriter w m => (w -> b) -> m a -> m (Tuple a b)

Projects a value from modifications made to the accumulator during an action.

#censor

censor :: forall w m a. MonadWriter w m => (w -> w) -> m a -> m a

Modify the final accumulator value by applying a function.

Modules