Copyright  (c) Andy Gill 2001 (c) Oregon Graduate Institute of Science and Technology 2001 

License  BSDstyle (see the file LICENSE) 
Maintainer  libraries@haskell.org 
Stability  experimental 
Portability  nonportable (multiparam classes, functional dependencies) 
Safe Haskell  Safe 
Language  Haskell2010 
Strict writer monads.
Inspired by the paper Functional Programming with Overloading and HigherOrder Polymorphism, Mark P Jones (http://web.cecs.pdx.edu/~mpj/pubs/springschool.html) Advanced School of Functional Programming, 1995.
Synopsis
 class (Monoid w, Monad m) => MonadWriter w m  m > w where
 listens :: MonadWriter w m => (w > b) > m a > m (a, b)
 censor :: MonadWriter w m => (w > w) > m a > m a
 type Writer w = WriterT w Identity
 runWriter :: Writer w a > (a, w)
 execWriter :: Writer w a > w
 mapWriter :: ((a, w) > (b, w')) > Writer w a > Writer w' b
 newtype WriterT w (m :: Type > Type) a = WriterT {
 runWriterT :: m (a, w)
 execWriterT :: Monad m => WriterT w m a > m w
 mapWriterT :: (m (a, w) > n (b, w')) > WriterT w m a > WriterT w' n b
 module Control.Monad.Trans
MonadWriter class
class (Monoid w, Monad m) => MonadWriter w m  m > w where Source #
writer :: (a, w) > m a Source #
embeds a simple writer action.writer
(a,w)
is an action that produces the output tell
ww
.
listen :: m a > m (a, w) Source #
is an action that executes the action listen
mm
and adds
its output to the value of the computation.
pass :: m (a, w > w) > m a Source #
is an action that executes the action pass
mm
, 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) Source #  
Monoid w => MonadWriter w ((,) w) Source #  Since: mtl2.2.2 
(Monoid w', MonadWriter w m) => MonadWriter w (AccumT w' m) Source #  There are two valid instances for
This instance chooses (1), reflecting that the intent
of Since: mtl2.3 
MonadWriter w m => MonadWriter w (ExceptT e m) Source #  Since: mtl2.2 
MonadWriter w m => MonadWriter w (IdentityT m) Source #  
MonadWriter w m => MonadWriter w (ReaderT r m) Source #  
MonadWriter w m => MonadWriter w (StateT s m) Source #  
MonadWriter w m => MonadWriter w (StateT s m) Source #  
(Monoid w, Monad m) => MonadWriter w (WriterT w m) Source #  Since: mtl2.3 
(Monoid w, Monad m) => MonadWriter w (WriterT w m) Source #  
(Monoid w, Monad m) => MonadWriter w (WriterT w m) Source #  
(Monoid w, Monad m) => MonadWriter w (RWST r w s m) Source #  Since: mtl2.3 
(Monoid w, Monad m) => MonadWriter w (RWST r w s m) Source #  
(Monoid w, Monad m) => MonadWriter w (RWST r w s m) Source #  
listens :: MonadWriter w m => (w > b) > m a > m (a, b) Source #
censor :: MonadWriter w m => (w > w) > m a > m a Source #
The Writer monad
runWriter :: Writer w a > (a, w) Source #
Unwrap a writer computation as a (result, output) pair.
(The inverse of writer
.)
execWriter :: Writer w a > w Source #
Extract the output from a writer computation.
execWriter
m =snd
(runWriter
m)
The WriterT monad transformer
newtype WriterT w (m :: Type > Type) a Source #
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
.
WriterT  

Instances
(MonadAccum w' m, Monoid w) => MonadAccum w' (WriterT w m) Source #  Since: mtl2.3 
(Monoid w, MonadError e m) => MonadError e (WriterT w m) Source #  
Defined in Control.Monad.Error.Class throwError :: e > WriterT w m a Source # catchError :: WriterT w m a > (e > WriterT w m a) > WriterT w m a Source #  
(Monoid w, MonadReader r m) => MonadReader r (WriterT w m) Source #  
(MonadSelect w' m, Monoid w) => MonadSelect w' (WriterT w m) Source #  'Readerizes' the writer: the 'ranking' function can see the value
that's been accumulated (of type Since: mtl2.3 
Defined in Control.Monad.Select  
(Monoid w, MonadState s m) => MonadState s (WriterT w m) Source #  
(Monoid w, Monad m) => MonadWriter w (WriterT w m) Source #  
Monoid w => MonadTrans (WriterT w)  
(Monoid w, MonadFail m) => MonadFail (WriterT w m)  
(Monoid w, MonadFix m) => MonadFix (WriterT w m)  
(Monoid w, MonadIO m) => MonadIO (WriterT w m)  
(Monoid w, MonadZip m) => MonadZip (WriterT w m)  
Foldable f => Foldable (WriterT w f)  
Defined in Control.Monad.Trans.Writer.Strict fold :: Monoid m => WriterT w f m > m Source # foldMap :: Monoid m => (a > m) > WriterT w f a > m Source # foldMap' :: Monoid m => (a > m) > WriterT w f a > m Source # foldr :: (a > b > b) > b > WriterT w f a > b Source # foldr' :: (a > b > b) > b > WriterT w f a > b Source # foldl :: (b > a > b) > b > WriterT w f a > b Source # foldl' :: (b > a > b) > b > WriterT w f a > b Source # foldr1 :: (a > a > a) > WriterT w f a > a Source # foldl1 :: (a > a > a) > WriterT w f a > a Source # toList :: WriterT w f a > [a] Source # null :: WriterT w f a > Bool Source # length :: WriterT w f a > Int Source # elem :: Eq a => a > WriterT w f a > Bool Source # maximum :: Ord a => WriterT w f a > a Source # minimum :: Ord a => WriterT w f a > a Source #  
(Eq w, Eq1 m) => Eq1 (WriterT w m)  
(Ord w, Ord1 m) => Ord1 (WriterT w m)  
Defined in Control.Monad.Trans.Writer.Strict  
(Read w, Read1 m) => Read1 (WriterT w m)  
Defined in Control.Monad.Trans.Writer.Strict liftReadsPrec :: (Int > ReadS a) > ReadS [a] > Int > ReadS (WriterT w m a) Source # liftReadList :: (Int > ReadS a) > ReadS [a] > ReadS [WriterT w m a] Source # liftReadPrec :: ReadPrec a > ReadPrec [a] > ReadPrec (WriterT w m a) Source # liftReadListPrec :: ReadPrec a > ReadPrec [a] > ReadPrec [WriterT w m a] Source #  
(Show w, Show1 m) => Show1 (WriterT w m)  
Contravariant m => Contravariant (WriterT w m)  
Traversable f => Traversable (WriterT w f)  
Defined in Control.Monad.Trans.Writer.Strict traverse :: Applicative f0 => (a > f0 b) > WriterT w f a > f0 (WriterT w f b) Source # sequenceA :: Applicative f0 => WriterT w f (f0 a) > f0 (WriterT w f a) Source # mapM :: Monad m => (a > m b) > WriterT w f a > m (WriterT w f b) Source # sequence :: Monad m => WriterT w f (m a) > m (WriterT w f a) Source #  
(Monoid w, Alternative m) => Alternative (WriterT w m)  
(Monoid w, Applicative m) => Applicative (WriterT w m)  
Defined in Control.Monad.Trans.Writer.Strict pure :: a > WriterT w m a Source # (<*>) :: WriterT w m (a > b) > WriterT w m a > WriterT w m b Source # liftA2 :: (a > b > c) > WriterT w m a > WriterT w m b > WriterT w m c Source # (*>) :: WriterT w m a > WriterT w m b > WriterT w m b Source # (<*) :: WriterT w m a > WriterT w m b > WriterT w m a Source #  
Functor m => Functor (WriterT w m)  
(Monoid w, Monad m) => Monad (WriterT w m)  
(Monoid w, MonadPlus m) => MonadPlus (WriterT w m)  
(Monoid w, MonadCont m) => MonadCont (WriterT w m) Source #  
Generic (WriterT w m a)  
(Read w, Read1 m, Read a) => Read (WriterT w m a)  
(Show w, Show1 m, Show a) => Show (WriterT w m a)  
(Eq w, Eq1 m, Eq a) => Eq (WriterT w m a)  
(Ord w, Ord1 m, Ord a) => Ord (WriterT w m a)  
Defined in Control.Monad.Trans.Writer.Strict compare :: WriterT w m a > WriterT w m a > Ordering # (<) :: WriterT w m a > WriterT w m a > Bool # (<=) :: WriterT w m a > WriterT w m a > Bool # (>) :: WriterT w m a > WriterT w m a > Bool # (>=) :: WriterT w m a > WriterT w m a > Bool #  
type Rep (WriterT w m a)  
Defined in Control.Monad.Trans.Writer.Strict 
execWriterT :: Monad m => WriterT w m a > m w Source #
Extract the output from a writer computation.
execWriterT
m =liftM
snd
(runWriterT
m)
mapWriterT :: (m (a, w) > n (b, w')) > WriterT w m a > WriterT w' n b Source #
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