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 | 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 | 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 >>=
combines the outputs of the subcomputations using mappend
.
WriterT (m (a, w)) |
Instances
(MonadAccum w' m, Monoid w) => MonadAccum w' (WriterT w m) Source # | Since: mtl-2.3 |
(Monoid w, MonadError e m) => MonadError e (WriterT w m) Source # | |
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 # | |
(MonadSelect w' m, Monoid w) => 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 # | |
(Monoid w, Monad m) => MonadWriter w (WriterT w m) Source # | |
Monoid w => MonadTrans (WriterT w) | |
(Monoid w, MonadFail m) => MonadFail (WriterT w m) | |
(Monoid w, MonadFix m) => MonadFix (WriterT w m) | |
(Monoid w, MonadIO m) => MonadIO (WriterT w m) | |
(Monoid w, MonadZip m) => MonadZip (WriterT w m) | |
Foldable f => Foldable (WriterT w f) | |
Defined in Control.Monad.Trans.Writer.Lazy fold :: Monoid m => WriterT w f m -> m Source # foldMap :: Monoid m => (a -> m) -> WriterT w f a -> m Source # foldMap' :: Monoid m => (a -> m) -> WriterT w f a -> m Source # foldr :: (a -> b -> b) -> b -> WriterT w f a -> b Source # foldr' :: (a -> b -> b) -> b -> WriterT w f a -> b Source # foldl :: (b -> a -> b) -> b -> WriterT w f a -> b Source # foldl' :: (b -> a -> b) -> b -> WriterT w f a -> b Source # foldr1 :: (a -> a -> a) -> WriterT w f a -> a Source # foldl1 :: (a -> a -> a) -> WriterT w f a -> a Source # toList :: WriterT w f a -> [a] Source # null :: WriterT w f a -> Bool Source # length :: WriterT w f a -> Int Source # elem :: Eq a => a -> WriterT w f a -> Bool Source # maximum :: Ord a => WriterT w f a -> a Source # minimum :: Ord a => WriterT w f a -> a Source # | |
(Eq w, Eq1 m) => Eq1 (WriterT w m) | |
(Ord w, Ord1 m) => Ord1 (WriterT w m) | |
Defined in Control.Monad.Trans.Writer.Lazy | |
(Read w, Read1 m) => Read1 (WriterT w m) | |
Defined in Control.Monad.Trans.Writer.Lazy liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (WriterT w m a) Source # liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [WriterT w m a] Source # liftReadPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec (WriterT w m a) Source # liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [WriterT w m a] Source # | |
(Show w, Show1 m) => Show1 (WriterT w m) | |
Contravariant m => Contravariant (WriterT w m) | |
Traversable f => Traversable (WriterT w f) | |
Defined in Control.Monad.Trans.Writer.Lazy traverse :: Applicative f0 => (a -> f0 b) -> WriterT w f a -> f0 (WriterT w f b) Source # sequenceA :: Applicative f0 => WriterT w f (f0 a) -> f0 (WriterT w f a) Source # mapM :: Monad m => (a -> m b) -> WriterT w f a -> m (WriterT w f b) Source # sequence :: Monad m => WriterT w f (m a) -> m (WriterT w f a) Source # | |
(Monoid w, Alternative m) => Alternative (WriterT w m) | |
(Monoid w, Applicative m) => Applicative (WriterT w m) | |
Defined in Control.Monad.Trans.Writer.Lazy pure :: a -> WriterT w m a Source # (<*>) :: WriterT w m (a -> b) -> WriterT w m a -> WriterT w m b Source # liftA2 :: (a -> b -> c) -> WriterT w m a -> WriterT w m b -> WriterT w m c Source # (*>) :: WriterT w m a -> WriterT w m b -> WriterT w m b Source # (<*) :: WriterT w m a -> WriterT w m b -> WriterT w m a Source # | |
Functor m => Functor (WriterT w m) | |
(Monoid w, Monad m) => Monad (WriterT w m) | |
(Monoid w, MonadPlus m) => MonadPlus (WriterT w m) | |
(Monoid w, MonadCont m) => MonadCont (WriterT w m) Source # | |
Generic (WriterT w m a) | |
(Read w, Read1 m, Read a) => Read (WriterT w m a) | |
(Show w, Show1 m, Show a) => Show (WriterT w m a) | |
(Eq w, Eq1 m, Eq a) => Eq (WriterT w m a) | |
(Ord w, Ord1 m, Ord a) => Ord (WriterT w m a) | |
Defined in Control.Monad.Trans.Writer.Lazy compare :: WriterT w m a -> WriterT w m a -> Ordering # (<) :: WriterT w m a -> WriterT w m a -> Bool # (<=) :: WriterT w m a -> WriterT w m a -> Bool # (>) :: WriterT w m a -> WriterT w m a -> Bool # (>=) :: WriterT w m a -> WriterT w m a -> Bool # | |
type Rep (WriterT w m a) | |
Defined in Control.Monad.Trans.Writer.Lazy |
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