Control.Monad.Writer.Class · mtl-2.3.1

classclass (Monoid w, Monad m) => MonadWriter w (m :: Type -> Type) | m -> w where

Methods

writer :: (a, w) -> m a
writer (a,w) embeds a simple writer action.
tell :: w -> m ()
tell w is an action that produces the output w.
listen :: m a -> m (a, w)
listen m is an action that executes the action m and adds its output to the value of the computation.
pass :: m (a, w -> w) -> m a
pass m is an action that executes the action m, which returns a value and a function, and returns the value, applying the function to the output.

Instances14MonadWriter, …

Monoid w => MonadWriter w (Tuple2 w)Defined in mtl-2.3.1 · Control.Monad.Writer.Class
MonadWriter w m => MonadWriter w (MaybeT m)Defined in mtl-2.3.1 · Control.Monad.Writer.Class
MonadWriter w m => MonadWriter w (ExceptT e m)Defined in mtl-2.3.1 · Control.Monad.Writer.Class
MonadWriter w m => MonadWriter w (IdentityT m)Defined in mtl-2.3.1 · Control.Monad.Writer.Class
MonadWriter w m => MonadWriter w (ReaderT r m)Defined in mtl-2.3.1 · Control.Monad.Writer.Class
MonadWriter w m => MonadWriter w (StateT s m)Defined in mtl-2.3.1 · Control.Monad.Writer.Class
MonadWriter w m => MonadWriter w (StateT s m)Defined in mtl-2.3.1 · Control.Monad.Writer.Class
(Monoid w, Monad m) => MonadWriter w (WriterT w m)Defined in mtl-2.3.1 · Control.Monad.Writer.Class
(Monoid w, Monad m) => MonadWriter w (WriterT w m)Defined in mtl-2.3.1 · Control.Monad.Writer.Class
(Monoid w, Monad m) => MonadWriter w (WriterT w m)Defined in mtl-2.3.1 · Control.Monad.Writer.Class
(Monoid w', MonadWriter w m) => MonadWriter w (AccumT w' m)Defined in mtl-2.3.1 · Control.Monad.Writer.Class
There are two valid instances for AccumT. It could either:
1. Lift the operations to the inner MonadWriter
2. Handle the operations itself, à la a WriterT.
This instance chooses (1), reflecting that the intent of AccumT as a type is different than that of WriterT.
(Monoid w, Monad m) => MonadWriter w (RWST r w s m)Defined in mtl-2.3.1 · Control.Monad.Writer.Class
(Monoid w, Monad m) => MonadWriter w (RWST r w s m)Defined in mtl-2.3.1 · Control.Monad.Writer.Class
(Monoid w, Monad m) => MonadWriter w (RWST r w s m)Defined in mtl-2.3.1 · Control.Monad.Writer.Class

valuelistens :: MonadWriter w m => (w -> b) -> m a -> m (a, b)

listens f m is an action that executes the action m and adds the result of applying f to the output to the value of the computation.

listens f m = liftM (id *** f) (listen m)

valuecensor :: MonadWriter w m => (w -> w) -> m a -> m a

#

censor f m is an action that executes the action m and applies the function f to its output, leaving the return value unchanged.

censor f m = pass (liftM (\x -> (x,f)) m)