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 |
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 :: 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 :: 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.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 :: 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 m
combines the outputs of the subcomputations using >>=
kmappend
(also
known as <>
):
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.
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.Trans