Module

Control.Monad.Writer.Class

This module defines the MonadWriter type class and its instances.

#MonadTell 

class (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 (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