Control.Monad.Writer.Class

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

• tell :: w -> m Unit

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

• listen :: forall a. m a -> m (Tuple a w)
• pass :: forall a. m (Tuple a (w -> w)) -> m a

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

Modify the final accumulator value by applying a function.