Control.Monad.Writer.Class

The MonadWriter class.

Inspired by the paper /Functional Programming with Overloading and Higher-Order Polymorphism/, Mark P Jones (http://web.cecs.pdx.edu/~mpj/pubs/springschool.html) Advanced School of Functional Programming, 1995.

Documentation

class (Monoid (WriterType m), Monad m) => MonadWriter m where

Associated Types

type WriterType m

Methods

tell :: WriterType m -> m ()

Shout to the monad what you want to be heard. The monad carries this packet upwards, merging it if needed (hence the Monoid requirement).

listen :: m a -> m (a, WriterType m)

Listen to a monad acting, and return what the monad "said".

pass :: m (a, WriterType m -> WriterType m) -> m a

Provide a writer transformer which changes internals of the written object.

Instances

MonadWriter m => MonadWriter (MaybeT m)

Defined in Control.Monad.Writer.Class

Associated Types

type WriterType (MaybeT m)

Methods

tell :: WriterType (MaybeT m) -> MaybeT m ()

listen :: MaybeT m a -> MaybeT m (a, WriterType (MaybeT m))

pass :: MaybeT m (a, WriterType (MaybeT m) -> WriterType (MaybeT m)) -> MaybeT m a

MonadWriter m => MonadWriter (ExceptT e m)

Defined in Control.Monad.Writer.Class

Associated Types

type WriterType (ExceptT e m)

Methods

tell :: WriterType (ExceptT e m) -> ExceptT e m ()

listen :: ExceptT e m a -> ExceptT e m (a, WriterType (ExceptT e m))

pass :: ExceptT e m (a, WriterType (ExceptT e m) -> WriterType (ExceptT e m)) -> ExceptT e m a

MonadWriter m => MonadWriter (IdentityT m)

Defined in Control.Monad.Writer.Class

Associated Types

type WriterType (IdentityT m)

Methods

tell :: WriterType (IdentityT m) -> IdentityT m ()

listen :: IdentityT m a -> IdentityT m (a, WriterType (IdentityT m))

pass :: IdentityT m (a, WriterType (IdentityT m) -> WriterType (IdentityT m)) -> IdentityT m a

MonadWriter m => MonadWriter (ReaderT r m)

Defined in Control.Monad.Writer.Class

Associated Types

type WriterType (ReaderT r m)

Methods

tell :: WriterType (ReaderT r m) -> ReaderT r m ()

listen :: ReaderT r m a -> ReaderT r m (a, WriterType (ReaderT r m))

pass :: ReaderT r m (a, WriterType (ReaderT r m) -> WriterType (ReaderT r m)) -> ReaderT r m a

MonadWriter m => MonadWriter (StateT s m)

Defined in Control.Monad.Writer.Class

Associated Types

type WriterType (StateT s m)

Methods

tell :: WriterType (StateT s m) -> StateT s m ()

listen :: StateT s m a -> StateT s m (a, WriterType (StateT s m))

pass :: StateT s m (a, WriterType (StateT s m) -> WriterType (StateT s m)) -> StateT s m a

MonadWriter m => MonadWriter (StateT s m)

Defined in Control.Monad.Writer.Class

Associated Types

type WriterType (StateT s m)

Methods

tell :: WriterType (StateT s m) -> StateT s m ()

listen :: StateT s m a -> StateT s m (a, WriterType (StateT s m))

pass :: StateT s m (a, WriterType (StateT s m) -> WriterType (StateT s m)) -> StateT s m a

(Monoid w, Monad m) => MonadWriter (WriterT w m)

Defined in Control.Monad.Writer.Class

Associated Types

type WriterType (WriterT w m)

Methods

tell :: WriterType (WriterT w m) -> WriterT w m ()

listen :: WriterT w m a -> WriterT w m (a, WriterType (WriterT w m))

pass :: WriterT w m (a, WriterType (WriterT w m) -> WriterType (WriterT w m)) -> WriterT w m a

(Monoid w, Monad m) => MonadWriter (WriterT w m)

Defined in Control.Monad.Writer.Class

Associated Types

type WriterType (WriterT w m)

Methods

tell :: WriterType (WriterT w m) -> WriterT w m ()

listen :: WriterT w m a -> WriterT w m (a, WriterType (WriterT w m))

pass :: WriterT w m (a, WriterType (WriterT w m) -> WriterType (WriterT w m)) -> WriterT w m a

(Monoid w, Monad m) => MonadWriter (RWST r w s m)

Defined in Control.Monad.Writer.Class

Associated Types

type WriterType (RWST r w s m)

Methods

tell :: WriterType (RWST r w s m) -> RWST r w s m ()

listen :: RWST r w s m a -> RWST r w s m (a, WriterType (RWST r w s m))

pass :: RWST r w s m (a, WriterType (RWST r w s m) -> WriterType (RWST r w s m)) -> RWST r w s m a

(Monoid w, Monad m) => MonadWriter (RWST r w s m)

Defined in Control.Monad.Writer.Class

Associated Types

type WriterType (RWST r w s m)

Methods

tell :: WriterType (RWST r w s m) -> RWST r w s m ()

listen :: RWST r w s m a -> RWST r w s m (a, WriterType (RWST r w s m))

pass :: RWST r w s m (a, WriterType (RWST r w s m) -> WriterType (RWST r w s m)) -> RWST r w s m a

listens :: MonadWriter m => (WriterType m -> 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)

censor :: MonadWriter m => (WriterType m -> WriterType m) -> 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)