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 | Haskell98 |
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.
- 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 a :: * -> (* -> *) -> * -> * = WriterT {
- runWriterT :: 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 #
embeds a simple writer action.writer
(a,w)
is an action that produces the output tell
ww
.
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 #
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.
MonadWriter w m => MonadWriter w (MaybeT m) # | |
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) # | |
(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) #
censor :: MonadWriter w m => (w -> w) -> m a -> m a #
The Writer monad
runWriter :: Writer w a -> (a, w) #
Unwrap a writer computation as a (result, output) pair.
(The inverse of writer
.)
execWriter :: Writer w a -> w #
Extract the output from a writer computation.
execWriter
m =snd
(runWriter
m)
The WriterT monad transformer
newtype WriterT w m 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
.
WriterT | |
|
runWriterT :: WriterT w m a -> m (a, w) #
execWriterT :: Monad m => WriterT w m a -> m w #
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 #
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