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.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.
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
- module Control.Monad.Fix
- module Control.Monad.Trans
- module Data.Monoid
MonadWriter class
class ( Monoid w, Monad m) => MonadWriter w m | m -> w where Source #
Methods
writer :: (a, w) -> m a Source #
embeds a simple writer action.
writer
(a,w)
is an action that produces the output
tell
w
w
.
listen :: m a -> m (a, w) Source #
is an action that executes the action
listen
m
m
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
m
m
, 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 # |
NOTE
: This instance is only defined for
Since: 2.2.2 |
MonadWriter w m => MonadWriter w ( StateT s m) Source # | |
MonadWriter w m => MonadWriter w ( StateT s m) Source # | |
MonadWriter w m => MonadWriter w ( ReaderT r m) Source # | |
MonadWriter w m => MonadWriter w ( IdentityT m) Source # | |
MonadWriter w m => MonadWriter w ( ExceptT e m) Source # |
Since: 2.2 |
( Error e, MonadWriter w m) => MonadWriter w ( ErrorT e m) Source # | |
( 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 # | |
( 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
.
Constructors
WriterT | |
Fields
|
Instances
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