Copyright	(c) Andy Gill 2001 (c) Oregon Graduate Institute of Science and Technology 2001
License	BSD-style (see the file LICENSE)
Maintainer	R.Paterson@city.ac.uk
Stability	experimental
Portability	portable
Safe Haskell	Safe
Language	Haskell98

Control.Monad.Trans.Writer.Lazy

Contents

The Writer monad
The WriterT monad transformer
Writer operations
Lifting other operations

Description

The lazy WriterT monad transformer, which adds collection of outputs (such as a count or string output) to a given monad.

This monad transformer provides only limited access to the output during the computation. For more general access, use Control.Monad.Trans.State instead.

This version builds its output lazily; for a constant-space version with almost the same interface, see Control.Monad.Trans.Writer.CPS.

Synopsis

type Writer w = WriterT w Identity
writer :: Monad m => (a, w) -> WriterT w m a
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
tell :: Monad m => w -> WriterT w m ()
listen :: Monad m => WriterT w m a -> WriterT w m (a, w)
listens :: Monad m => (w -> b) -> WriterT w m a -> WriterT w m (a, b)
pass :: Monad m => WriterT w m (a, w -> w) -> WriterT w m a
censor :: Monad m => (w -> w) -> WriterT w m a -> WriterT w m a
liftCallCC :: Monoid w => CallCC m (a, w) (b, w) -> CallCC (WriterT w m) a b
liftCatch :: Catch e m (a, w) -> Catch e (WriterT w m) a

The Writer monad

type Writer w = WriterT w Identity Source #

A writer monad parameterized by the type w of output to accumulate.

The return function produces the output mempty, while >>= combines the outputs of the subcomputations using mappend.

writer :: Monad m => (a, w) -> WriterT w m a Source #

Construct a writer computation from a (result, output) pair. (The inverse of runWriter.)

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)
```

mapWriter :: ((a, w) -> (b, w')) -> Writer w a -> Writer w' b Source #

Map both the return value and output of a computation using the given function.

runWriter (mapWriter f m) = f (runWriter m)

The WriterT monad transformer

newtype WriterT w m 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 runWriterT :: m (a, w)

Instances

Instances details

Monoid w => MonadTrans (WriterT w) #
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods lift :: Monad m => m a -> WriterT w m a Source #
(Monoid w, Monad m) => Monad (WriterT w m) #
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods (>>=) :: WriterT w m a -> (a -> WriterT w m b) -> WriterT w m b Source # (>>) :: WriterT w m a -> WriterT w m b -> WriterT w m b Source # return :: a -> WriterT w m a Source #
Functor m => Functor (WriterT w m) #
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods fmap :: (a -> b) -> WriterT w m a -> WriterT w m b Source # (<$) :: a -> WriterT w m b -> WriterT w m a Source #
(Monoid w, MonadFix m) => MonadFix (WriterT w m) #
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods mfix :: (a -> WriterT w m a) -> WriterT w m a Source #
(Monoid w, MonadFail m) => MonadFail (WriterT w m) #
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods fail :: String -> WriterT w m a Source #
(Monoid w, Applicative m) => Applicative (WriterT w m) #
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods 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 #
Foldable f => Foldable (WriterT w f) #
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods 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 # sum :: Num a => WriterT w f a -> a Source # product :: Num a => WriterT w f a -> a Source #
Traversable f => Traversable (WriterT w f) #
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods 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 #
Contravariant m => Contravariant (WriterT w m) #
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods contramap :: (a -> b) -> WriterT w m b -> WriterT w m a Source # (>$) :: b -> WriterT w m b -> WriterT w m a Source #
(Eq w, Eq1 m) => Eq1 (WriterT w m) #
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods liftEq :: (a -> b -> Bool) -> WriterT w m a -> WriterT w m b -> Bool Source #
(Ord w, Ord1 m) => Ord1 (WriterT w m) #
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods liftCompare :: (a -> b -> Ordering) -> WriterT w m a -> WriterT w m b -> Ordering Source #
(Read w, Read1 m) => Read1 (WriterT w m) #
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods 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) #
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> WriterT w m a -> ShowS Source # liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [WriterT w m a] -> ShowS Source #
(Monoid w, MonadZip m) => MonadZip (WriterT w m) #
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods mzip :: WriterT w m a -> WriterT w m b -> WriterT w m (a, b) Source # mzipWith :: (a -> b -> c) -> WriterT w m a -> WriterT w m b -> WriterT w m c Source # munzip :: WriterT w m (a, b) -> (WriterT w m a, WriterT w m b) Source #
(Monoid w, MonadIO m) => MonadIO (WriterT w m) #
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods liftIO :: IO a -> WriterT w m a Source #
(Monoid w, Alternative m) => Alternative (WriterT w m) #
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods empty :: WriterT w m a Source # (<\|>) :: WriterT w m a -> WriterT w m a -> WriterT w m a Source # some :: WriterT w m a -> WriterT w m [a] Source # many :: WriterT w m a -> WriterT w m [a] Source #
(Monoid w, MonadPlus m) => MonadPlus (WriterT w m) #
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods mzero :: WriterT w m a Source # mplus :: WriterT w m a -> WriterT w m a -> WriterT w m a Source #
(Eq w, Eq1 m, Eq a) => Eq (WriterT w m a) #
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods (==) :: WriterT w m a -> WriterT w m a -> Bool # (/=) :: WriterT w m a -> WriterT w m a -> Bool #
(Ord w, Ord1 m, Ord a) => Ord (WriterT w m a) #
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods 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 # max :: WriterT w m a -> WriterT w m a -> WriterT w m a # min :: WriterT w m a -> WriterT w m a -> WriterT w m a #
(Read w, Read1 m, Read a) => Read (WriterT w m a) #
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods readsPrec :: Int -> ReadS (WriterT w m a) Source # readList :: ReadS [WriterT w m a] Source # readPrec :: ReadPrec (WriterT w m a) Source # readListPrec :: ReadPrec [WriterT w m a] Source #
(Show w, Show1 m, Show a) => Show (WriterT w m a) #
Instance details Defined in Control.Monad.Trans.Writer.Lazy Methods showsPrec :: Int -> WriterT w m a -> ShowS Source # show :: WriterT w m a -> String Source # showList :: [WriterT w m a] -> ShowS 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)

Writer operations

tell :: Monad m => w -> WriterT w m () Source #

tell w is an action that produces the output w.

listen :: Monad m => WriterT w m a -> WriterT w m (a, w) Source #

listen m is an action that executes the action m and adds its output to the value of the computation.

runWriterT (listen m) = liftM (\ (a, w) -> ((a, w), w)) (runWriterT m)

listens :: Monad m => (w -> b) -> WriterT w m a -> WriterT w m (a, b) Source #

listens f m is an action that executes the action m and adds the result of applying f to the output to the value of the computation.

listens f m = liftM (id *** f) (listen m)

runWriterT (listens f m) = liftM (\ (a, w) -> ((a, f w), w)) (runWriterT m)

pass :: Monad m => WriterT w m (a, w -> w) -> WriterT w m a Source #

pass m is an action that executes the action m, which returns a value and a function, and returns the value, applying the function to the output.

runWriterT (pass m) = liftM (\ ((a, f), w) -> (a, f w)) (runWriterT m)

censor :: Monad m => (w -> w) -> WriterT w m a -> WriterT w m a Source #

censor f m is an action that executes the action m and applies the function f to its output, leaving the return value unchanged.

censor f m = pass (liftM (\ x -> (x,f)) m)

runWriterT (censor f m) = liftM (\ (a, w) -> (a, f w)) (runWriterT m)

Lifting other operations

liftCallCC :: Monoid w => CallCC m (a, w) (b, w) -> CallCC (WriterT w m) a b Source #

Lift a callCC operation to the new monad.

liftCatch :: Catch e m (a, w) -> Catch e (WriterT w m) a Source #

Lift a catchE operation to the new monad.