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

License  BSDstyle (see the file LICENSE) 
Maintainer  [email protected] 
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.
class (Monoid w, Monad m) => MonadWriter w m  m > w where Source
writer :: (a, w) > m a Source
writer (a,w)
embeds a simple writer action.
tell w
is an action that produces the output w
.
listen :: m a > m (a, w) Source
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 Source
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.
MonadWriter w m => MonadWriter w (MaybeT m)  
Monoid w => MonadWriter w ((,) w) 
NOTE: This instance is only defined for Since: mtl2.2.2 
MonadWriter w m => MonadWriter w (StateT s m)  
MonadWriter w m => MonadWriter w (StateT s m)  
MonadWriter w m => MonadWriter w (IdentityT m)  
MonadWriter w m => MonadWriter w (ExceptT e m)  Since: mtl2.2 
(Error e, MonadWriter w m) => MonadWriter w (ErrorT e m)  
(Monoid w, Monad m) => MonadWriter w (WriterT w m)  
(Monoid w, Monad m) => MonadWriter w (WriterT w m)  
MonadWriter w m => MonadWriter w (ReaderT r m)  
(Monoid w, Monad m) => MonadWriter w (RWST r w s m)  
(Monoid w, Monad m) => MonadWriter w (RWST r w s m)  
listens :: MonadWriter w m => (w > b) > m a > m (a, b) Source
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.
censor :: MonadWriter w m => (w > w) > m a > m a Source
censor f m
is an action that executes the action m
and applies the function f
to its output, leaving the return value unchanged.
type Writer w = WriterT w Identity Source
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 :: 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)
mapWriter :: ((a, w) > (b, w')) > Writer w a > Writer w' b Source
Map both the return value and output of a computation using the given function.
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  
Fields

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
module Control.Monad.Fix
module Control.Monad.Trans
module Data.Monoid
© The University of Glasgow and others
Licensed under a BSDstyle license (see top of the page).
https://downloads.haskell.org/~ghc/8.6.1/docs/html/libraries/mtl2.2.2/ControlMonadWriterStrict.html