Control.Monad.Writer.CPS

Strict writer monads that use continuation-passing-style to achieve constant space usage.

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.

Since: mtl-2.3, transformers-0.5.6

MonadWriter class

class (Monoid w, Monad m) => MonadWriter w m | m -> w where

Minimal complete definition

(writer | tell), listen, pass

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.

Instances

MonadWriter w m => MonadWriter w (MaybeT m)

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> MaybeT m a

tell :: w -> MaybeT m ()

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

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

Monoid w => MonadWriter w ((,) w)

Since: 2.2.2

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> (w, a)

tell :: w -> (w, ())

listen :: (w, a) -> (w, (a, w))

pass :: (w, (a, w -> w)) -> (w, a)

(Monoid w', MonadWriter w m) => MonadWriter w (AccumT w' m)

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.

Since: 2.3

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> AccumT w' m a

tell :: w -> AccumT w' m ()

listen :: AccumT w' m a -> AccumT w' m (a, w)

pass :: AccumT w' m (a, w -> w) -> AccumT w' m a

MonadWriter w m => MonadWriter w (ExceptT e m)

Since: 2.2

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> ExceptT e m a

tell :: w -> ExceptT e m ()

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

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

MonadWriter w m => MonadWriter w (IdentityT m)

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> IdentityT m a

tell :: w -> IdentityT m ()

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

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

MonadWriter w m => MonadWriter w (ReaderT r m)

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> ReaderT r m a

tell :: w -> ReaderT r m ()

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

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

MonadWriter w m => MonadWriter w (StateT s m)

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> StateT s m a

tell :: w -> StateT s m ()

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

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

MonadWriter w m => MonadWriter w (StateT s m)

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> StateT s m a

tell :: w -> StateT s m ()

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

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

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

Since: 2.3

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> WriterT w m a

tell :: w -> WriterT w m ()

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

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

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

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> WriterT w m a

tell :: w -> WriterT w m ()

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

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

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

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> WriterT w m a

tell :: w -> WriterT w m ()

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

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

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

Since: 2.3

Defined in Control.Monad.Writer.Class

Methods

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

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

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

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

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

Defined in Control.Monad.Writer.Class

Methods

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

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

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

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

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

Defined in Control.Monad.Writer.Class

Methods

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

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

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

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

listens :: 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)

censor :: 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)

The Writer monad

type Writer w = WriterT w Identity

A writer monad parameterized by the type w of output to accumulate.

The return function produces the output mempty, while >>= combines the outputs of the subcomputations using mappend.

runWriter :: Monoid w => Writer w a -> (a, w)

Unwrap a writer computation as a (result, output) pair. (The inverse of writer.)

execWriter :: Monoid w => Writer w a -> w

Extract the output from a writer computation.

• execWriter m = snd (runWriter m)

mapWriter :: (Monoid w, Monoid w') => ((a, w) -> (b, w')) -> Writer w a -> Writer w' b

Map both the return value and output of a computation using the given function.

• runWriter (mapWriter f m) = f (runWriter m)

The WriterT monad transformer

data WriterT w (m :: Type -> Type) a

A writer monad parameterized by:

• w - the output to accumulate.
• m - The inner monad.

The return function produces the output mempty, while >>= combines the outputs of the subcomputations using mappend.

Instances

MonadAccum w' m => MonadAccum w' (WriterT w m)

Since: 2.3

Defined in Control.Monad.Accum

Methods

look :: WriterT w m w'

add :: w' -> WriterT w m ()

accum :: (w' -> (a, w')) -> WriterT w m a

(Monoid w, MonadError e m) => MonadError e (WriterT w m)

Since: 2.3

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> WriterT w m a

catchError :: WriterT w m a -> (e -> WriterT w m a) -> WriterT w m a

(Monoid w, MonadReader r m) => MonadReader r (WriterT w m)

Since: 2.3

Defined in Control.Monad.Reader.Class

Methods

ask :: WriterT w m r

local :: (r -> r) -> WriterT w m a -> WriterT w m a

reader :: (r -> a) -> WriterT w m a

MonadSelect w' m => MonadSelect w' (WriterT w m)

'Readerizes' the writer: the 'ranking' function can see the value that's been accumulated (of type w), but can't add anything to the log. Effectively, can be thought of as 'extending' the 'ranking' by all values of w, but which w gets given to any rank calls is predetermined by the 'outer writer' (and cannot change).

Since: 2.3

Defined in Control.Monad.Select

Methods

select :: ((a -> w') -> a) -> WriterT w m a

(Monoid w, MonadState s m) => MonadState s (WriterT w m)

Since: 2.3

Defined in Control.Monad.State.Class

Methods

get :: WriterT w m s

put :: s -> WriterT w m ()

state :: (s -> (a, s)) -> WriterT w m a

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

Since: 2.3

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> WriterT w m a

tell :: w -> WriterT w m ()

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

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

MonadTrans (WriterT w)

Defined in Control.Monad.Trans.Writer.CPS

Methods

lift :: Monad m => m a -> WriterT w m a

MonadFail m => MonadFail (WriterT w m)

Defined in Control.Monad.Trans.Writer.CPS

Methods

fail :: String -> WriterT w m a

MonadFix m => MonadFix (WriterT w m)

Defined in Control.Monad.Trans.Writer.CPS

Methods

mfix :: (a -> WriterT w m a) -> WriterT w m a

MonadIO m => MonadIO (WriterT w m)

Defined in Control.Monad.Trans.Writer.CPS

Methods

liftIO :: IO a -> WriterT w m a

(Functor m, MonadPlus m) => Alternative (WriterT w m)

Defined in Control.Monad.Trans.Writer.CPS

Methods

empty :: WriterT w m a

(<|>) :: WriterT w m a -> WriterT w m a -> WriterT w m a

some :: WriterT w m a -> WriterT w m [a]

many :: WriterT w m a -> WriterT w m [a]

(Functor m, Monad m) => Applicative (WriterT w m)

Defined in Control.Monad.Trans.Writer.CPS

Methods

pure :: a -> WriterT w m a

(<*>) :: WriterT w m (a -> b) -> WriterT w m a -> WriterT w m b

liftA2 :: (a -> b -> c) -> WriterT w m a -> WriterT w m b -> WriterT w m c

(*>) :: WriterT w m a -> WriterT w m b -> WriterT w m b

(<*) :: WriterT w m a -> WriterT w m b -> WriterT w m a

Functor m => Functor (WriterT w m)

Defined in Control.Monad.Trans.Writer.CPS

Methods

fmap :: (a -> b) -> WriterT w m a -> WriterT w m b

(<$) :: a -> WriterT w m b -> WriterT w m a

Monad m => Monad (WriterT w m)

Defined in Control.Monad.Trans.Writer.CPS

Methods

(>>=) :: WriterT w m a -> (a -> WriterT w m b) -> WriterT w m b

(>>) :: WriterT w m a -> WriterT w m b -> WriterT w m b

return :: a -> WriterT w m a

(Functor m, MonadPlus m) => MonadPlus (WriterT w m)

Defined in Control.Monad.Trans.Writer.CPS

Methods

mzero :: WriterT w m a

mplus :: WriterT w m a -> WriterT w m a -> WriterT w m a

(Monoid w, MonadCont m) => MonadCont (WriterT w m)

Since: 2.3

Defined in Control.Monad.Cont.Class

Methods

callCC :: ((a -> WriterT w m b) -> WriterT w m a) -> WriterT w m a

execWriterT :: (Monad m, Monoid w) => WriterT w m a -> m w

Extract the output from a writer computation.

• execWriterT m = liftM snd (runWriterT m)

mapWriterT :: (Monad n, Monoid w, Monoid w') => (m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b

Map both the return value and output of a computation using the given function.

• runWriterT (mapWriterT f m) = f (runWriterT m)

module Control.Monad.Trans