Portability | non-portable (type families) |
---|---|
Stability | experimental |
Maintainer | libraries@haskell.org |
Control.Monad.Writer.Strict
Description
Strict 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 (WriterType m), Monad m) => MonadWriter m where
- type WriterType m
- tell :: WriterType m -> m ()
- listen :: m a -> m (a, WriterType m)
- pass :: m (a, WriterType m -> WriterType m) -> m a
- listens :: MonadWriter m => (WriterType m -> b) -> m a -> m (a, b)
- censor :: MonadWriter m => (WriterType m -> WriterType m) -> 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)
- 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 (WriterType m), Monad m) => MonadWriter m whereSource
Associated Types
type WriterType m Source
Methods
tell :: WriterType m -> m ()Source
listen :: m a -> m (a, WriterType m)Source
pass :: m (a, WriterType m -> WriterType m) -> m aSource
Instances
MonadWriter m => MonadWriter (MaybeT m) | |
MonadWriter m => MonadWriter (IdentityT m) | |
(Monoid w, Monad m) => MonadWriter (WriterT w m) | |
(Monoid w, Monad m) => MonadWriter (WriterT w m) | |
MonadWriter m => MonadWriter (StateT s m) | |
MonadWriter m => MonadWriter (StateT s m) | |
MonadWriter m => MonadWriter (ReaderT r m) | |
(Error e, MonadWriter m) => MonadWriter (ErrorT e m) | |
(Monoid w, Monad m) => MonadWriter (RWST r w s m) | |
(Monoid w, Monad m) => MonadWriter (RWST r w s m) |
listens :: MonadWriter m => (WriterType m -> b) -> m a -> m (a, b)Source
censor :: MonadWriter m => (WriterType m -> WriterType m) -> m a -> m aSource
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
.
Constructors
WriterT | |
Fields
|
Instances
Monoid w => MonadTrans (WriterT w) | |
(Monoid w, Monad m) => Monad (WriterT w m) | |
Functor m => Functor (WriterT w m) | |
(Monoid w, MonadFix m) => MonadFix (WriterT w m) | |
(Monoid w, MonadPlus m) => MonadPlus (WriterT w m) | |
(Monoid w, Applicative m) => Applicative (WriterT w m) | |
(Monoid w, Alternative m) => Alternative (WriterT w m) | |
(Monoid w, MonadIO m) => MonadIO (WriterT w m) | |
(Monoid w, MonadCont m) => MonadCont (WriterT w m) | |
(Monoid w, MonadError m) => MonadError (WriterT w m) | |
(Monoid w, MonadReader m) => MonadReader (WriterT w m) | |
(Monoid w, MonadState m) => MonadState (WriterT w m) | |
(Monoid w, Monad m) => MonadWriter (WriterT w m) |
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