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 |
Strict writer monads that use continuation-passing-style to achieve constant space usage.
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.
Since: mtl-2.3, transformers-0.5.6
Synopsis
- class (Monoid w, Monad m) => MonadWriter w (m :: Type -> Type) | 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 :: Monoid w => Writer w a -> (a, w)
- execWriter :: Monoid w => Writer w a -> w
- mapWriter :: (Monoid w, Monoid w') => ((a, w) -> (b, w')) -> Writer w a -> Writer w' b
- data WriterT w (m :: Type -> Type) a
- execWriterT :: (Monad m, Monoid w) => WriterT w m a -> m w
- mapWriterT :: (Monad n, Monoid w, Monoid w') => (m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b
- module Control.Monad.Trans
MonadWriter class
class (Monoid w, Monad m) => MonadWriter w (m :: Type -> Type) | m -> w where Source #
writer :: (a, w) -> m a Source #
embeds a simple writer action.writer
(a,w)
is an action that produces the output tell
ww
.
listen :: m a -> m (a, w) Source #
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 Source #
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.
Instances
MonadWriter w m => MonadWriter w (MaybeT m) Source # | |
Monoid w => MonadWriter w ((,) w) Source # | Since: mtl-2.2.2 |
(Monoid w', MonadWriter w m) => MonadWriter w (AccumT w' m) Source # | There are two valid instances for
This instance chooses (1), reflecting that the intent
of Since: mtl-2.3 |
MonadWriter w m => MonadWriter w (ExceptT e m) Source # | Since: mtl-2.2 |
MonadWriter w m => MonadWriter w (IdentityT m) Source # | |
MonadWriter w m => MonadWriter w (ReaderT r m) Source # | |
MonadWriter w m => MonadWriter w (StateT s m) Source # | |
MonadWriter w m => MonadWriter w (StateT s m) Source # | |
(Monoid w, Monad m) => MonadWriter w (WriterT w m) Source # | Since: mtl-2.3 |
(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 # | Since: mtl-2.3 |
(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 :: Monoid w => Writer w a -> (a, w) Source #
Unwrap a writer computation as a (result, output) pair.
(The inverse of writer
.)
execWriter :: Monoid w => Writer w a -> w Source #
Extract the output from a writer computation.
execWriter
m =snd
(runWriter
m)
The WriterT monad transformer
data 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 m
combines the outputs of the subcomputations using >>=
kmappend
(also
known as <>
):
Instances
MonadAccum w' m => MonadAccum w' (WriterT w m) Source # | Since: mtl-2.3 | ||||
(Monoid w, MonadError e m) => MonadError e (WriterT w m) Source # | Since: mtl-2.3 | ||||
Defined in Control.Monad.Error.Class throwError :: e -> WriterT w m a Source # catchError :: WriterT w m a -> (e -> WriterT w m a) -> WriterT w m a Source # | |||||
(Monoid w, MonadReader r m) => MonadReader r (WriterT w m) Source # | Since: mtl-2.3 | ||||
MonadSelect w' m => MonadSelect w' (WriterT w m) Source # | 'Readerizes' the writer: the 'ranking' function can see the value
that's been accumulated (of type Since: mtl-2.3 | ||||
Defined in Control.Monad.Select | |||||
(Monoid w, MonadState s m) => MonadState s (WriterT w m) Source # | Since: mtl-2.3 | ||||
(Monoid w, Monad m) => MonadWriter w (WriterT w m) Source # | Since: mtl-2.3 | ||||
MonadTrans (WriterT w) Source # | |||||
(Functor m, MonadPlus m) => Alternative (WriterT w m) Source # | |||||
(Functor m, Monad m) => Applicative (WriterT w m) Source # | |||||
Defined in Control.Monad.Trans.Writer.CPS | |||||
Functor m => Functor (WriterT w m) Source # | |||||
Monad m => Monad (WriterT w m) Source # | |||||
(Functor m, MonadPlus m) => MonadPlus (WriterT w m) Source # | |||||
MonadFail m => MonadFail (WriterT w m) Source # | |||||
Defined in Control.Monad.Trans.Writer.CPS | |||||
MonadFix m => MonadFix (WriterT w m) Source # | |||||
Defined in Control.Monad.Trans.Writer.CPS | |||||
MonadIO m => MonadIO (WriterT w m) Source # | |||||
Defined in Control.Monad.Trans.Writer.CPS | |||||
(Monoid w, MonadCont m) => MonadCont (WriterT w m) Source # | Since: mtl-2.3 | ||||
Generic (WriterT w m a) Source # | |||||
Defined in Control.Monad.Trans.Writer.CPS
| |||||
type Rep (WriterT w m a) Source # | |||||
Defined in Control.Monad.Trans.Writer.CPS |
execWriterT :: (Monad m, Monoid w) => WriterT w m a -> m w Source #
Extract the output from a writer computation.
execWriterT
m =liftM
snd
(runWriterT
m)
mapWriterT :: (Monad n, Monoid w, Monoid w') => (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.Trans