| Copyright | (c) Andy Gill 2001 (c) Oregon Graduate Institute of Science and Technology 2001 | 
|---|---|
| License | BSD-style (see the file LICENSE) | 
| Maintainer | libraries@haskell.org | 
| Stability | experimental | 
| Portability | non-portable (multi-param classes, functional dependencies) | 
| Safe Haskell | Safe | 
| Language | Haskell2010 | 
Control.Monad.Writer.Lazy
Description
Lazy writer monads.
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.
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 (m (a, w))
- runWriterT :: WriterT w m a -> 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
- module Control.Monad.Fix
- module Control.Monad.Trans
- module Data.Monoid
MonadWriter class
class (Monoid w, Monad m) => MonadWriter w m | m -> w where #
Methods
writer (a,w)
tell ww.
listen mm and adds
 its output to the value of the computation.
pass :: m (a, w -> w) -> m a #
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) # | |
| Monoid w => MonadWriter w ((,) w) # | NOTE: This instance is only defined for  Since: mtl-2.2.2 | 
| (Error e, MonadWriter w m) => MonadWriter w (ErrorT e m) # | |
| MonadWriter w m => MonadWriter w (ExceptT e m) # | Since: mtl-2.2 | 
| MonadWriter w m => MonadWriter w (IdentityT m) # | |
| MonadWriter w m => MonadWriter w (ReaderT r m) # | |
| MonadWriter w m => MonadWriter w (StateT s m) # | |
| MonadWriter w m => MonadWriter w (StateT s m) # | |
| (Monoid w, Monad m) => MonadWriter w (WriterT w m) # | |
| (Monoid w, Monad m) => MonadWriter w (WriterT w 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) #
censor :: MonadWriter w m => (w -> w) -> m a -> m a #
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.
- execWriterm =- snd(- runWriterm)
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.
Constructors
| WriterT (m (a, w)) | 
Instances
runWriterT :: WriterT w m a -> m (a, w) Source #
execWriterT :: Monad m => WriterT w m a -> m w Source #
Extract the output from a writer computation.
- execWriterTm =- liftM- snd(- runWriterTm)
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(- mapWriterTf m) = f (- runWriterTm)
module Control.Monad
module Control.Monad.Fix
module Control.Monad.Trans
module Data.Monoid