Control.Monad.Writer

type Writer w = WriterT w Identity

The Writer monad is a synonym for the WriterT monad transformer, applied to the Identity monad.

writer :: forall a w. Tuple a w -> Writer w a

Creates a Writer from a result and output pair.

runWriter :: forall a w. Writer w a -> Tuple a w

Run a computation in the Writer monad

execWriter :: forall a w. Writer w a -> w

Run a computation in the Writer monad, discarding the result

mapWriter :: forall b a w2 w1. (Tuple a w1 -> Tuple b w2) -> Writer w1 a -> Writer w2 b

Change the result and accumulator types in a Writer monad action

class (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 (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 b a m w. 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 a m w. MonadWriter w m => (w -> w) -> m a -> m a

Modify the final accumulator value by applying a function.

newtype 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

• WriterT (m (Tuple a w))

Instances

• Newtype (WriterT w m a) _
• (Functor m) => Functor (WriterT w m)
• (Semigroup w, Apply m) => Apply (WriterT w m)
• (Monoid w, Applicative m) => Applicative (WriterT w m)
• (Alt m) => Alt (WriterT w m)
• (Plus m) => Plus (WriterT w m)
• (Monoid w, Alternative m) => Alternative (WriterT w m)
• (Semigroup w, Bind m) => Bind (WriterT w m)
• (Monoid w, Monad m) => Monad (WriterT w m)
• (Monoid w, MonadRec m) => MonadRec (WriterT w m)
• (Monoid w, MonadZero m) => MonadZero (WriterT w m)
• (Monoid w, MonadPlus m) => MonadPlus (WriterT w m)
• (Monoid w) => MonadTrans (WriterT w)
• (Monoid w, MonadEffect m) => MonadEffect (WriterT w m)
• (Monoid w, MonadCont m) => MonadCont (WriterT w m)
• (Monoid w, MonadThrow e m) => MonadThrow e (WriterT w m)
• (Monoid w, MonadError e m) => MonadError e (WriterT w m)
• (Monoid w, MonadAsk r m) => MonadAsk r (WriterT w m)
• (Monoid w, MonadReader r m) => MonadReader r (WriterT w m)
• (Monoid w, MonadState s m) => MonadState s (WriterT w m)
• (Monoid w, Monad m) => MonadTell w (WriterT w m)
• (Monoid w, Monad m) => MonadWriter w (WriterT w m)

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

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

Run a computation in the WriterT monad.

mapWriterT :: forall b a m2 m1 w2 w1. (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.

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

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