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.Strict

Contents

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

Description

The strict 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 strictly; for a lazy version with the same interface, see Control.Monad.Trans.Writer.Lazy. Although the output is built strictly, it is not possible to achieve constant space behaviour with this transformer: for that, use Control.Monad.Trans.State.Strict instead.

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

Monoid w => WriterT w) Source #

Instance details

Methods

lift :: Monad m => m a -> WriterT w m a Source #

(Monoid w, Monad m) => Monad (WriterT w m) Source #

Instance details

Methods

(>>=) :: WriterT w m a -> (a -> WriterT w m b) -> WriterT w m b #

(>>) :: WriterT w m a -> WriterT w m b -> WriterT w m b #

return :: a -> WriterT w m a #

fail :: String -> WriterT w m a #

Functor m => Functor (WriterT w m) Source #

Instance details

Methods

fmap :: (a -> b) -> WriterT w m a -> WriterT w m b #

(<$) :: a -> WriterT w m b -> WriterT w m a #

(Monoid w, MonadFix m) => MonadFix (WriterT w m) Source #

Instance details

Methods

mfix :: (a -> WriterT w m a) -> WriterT w m a #

(Monoid w, MonadFail m) => MonadFail (WriterT w m) Source #

Instance details

Methods

fail :: String -> WriterT w m a #

(Monoid w, Applicative m) => Applicative (WriterT w m) Source #

Instance details

Methods

pure :: a -> WriterT w m a #

(<*>) :: WriterT w m (a -> b) -> WriterT w m a -> WriterT w m b #

liftA2 :: (a -> b -> c) -> WriterT w m a -> WriterT w m b -> WriterT w m c #

(*>) :: WriterT w m a -> WriterT w m b -> WriterT w m b #

(<*) :: WriterT w m a -> WriterT w m b -> WriterT w m a #

Foldable f => Foldable (WriterT w f) Source #

Instance details

Methods

fold :: Monoid m => WriterT w f m -> m #

foldMap :: Monoid m => (a -> m) -> WriterT w f a -> m #

foldr :: (a -> b -> b) -> b -> WriterT w f a -> b #

foldr' :: (a -> b -> b) -> b -> WriterT w f a -> b #

foldl :: (b -> a -> b) -> b -> WriterT w f a -> b #

foldl' :: (b -> a -> b) -> b -> WriterT w f a -> b #

foldr1 :: (a -> a -> a) -> WriterT w f a -> a #

foldl1 :: (a -> a -> a) -> WriterT w f a -> a #

toList :: WriterT w f a -> [a] #

null :: WriterT w f a -> Bool #

length :: WriterT w f a -> Int #

elem :: Eq a => a -> WriterT w f a -> Bool #

maximum :: Ord a => WriterT w f a -> a #

minimum :: Ord a => WriterT w f a -> a #

sum :: Num a => WriterT w f a -> a #

product :: Num a => WriterT w f a -> a #

Traversable f => Traversable (WriterT w f) Source #

Instance details

Methods

traverse :: Applicative f0 => (a -> f0 b) -> WriterT w f a -> f0 (WriterT w f b) #

sequenceA :: Applicative f0 => WriterT w f (f0 a) -> f0 (WriterT w f a) #

mapM :: Monad m => (a -> m b) -> WriterT w f a -> m (WriterT w f b) #

sequence :: Monad m => WriterT w f (m a) -> m (WriterT w f a) #

(Eq w, Eq1 m) => Eq1 (WriterT w m) Source #

Instance details

Methods

liftEq :: (a -> b -> Bool) -> WriterT w m a -> WriterT w m b -> Bool #

(Ord w, Ord1 m) => Ord1 (WriterT w m) Source #

Instance details

Methods

liftCompare :: (a -> b -> Ordering) -> WriterT w m a -> WriterT w m b -> Ordering #

(Read w, Read1 m) => Read1 (WriterT w m) Source #

Instance details

Methods

liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (WriterT w m a) #

liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [WriterT w m a] #

liftReadPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec (WriterT w m a) #

liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [WriterT w m a] #

(Show w, Show1 m) => Show1 (WriterT w m) Source #

Instance details

Methods

liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> WriterT w m a -> ShowS #

liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [WriterT w m a] -> ShowS #

(Monoid w, MonadZip m) => MonadZip (WriterT w m) Source #

Instance details

Methods

mzip :: WriterT w m a -> WriterT w m b -> WriterT w m (a, b) #

mzipWith :: (a -> b -> c) -> WriterT w m a -> WriterT w m b -> WriterT w m c #

munzip :: WriterT w m (a, b) -> (WriterT w m a, WriterT w m b) #

(Monoid w, MonadIO m) => MonadIO (WriterT w m) Source #

Instance details

Methods

liftIO :: IO a -> WriterT w m a #

(Monoid w, Alternative m) => Alternative (WriterT w m) Source #

Instance details

Methods

empty :: WriterT w m a #

(<|>) :: WriterT w m a -> WriterT w m a -> WriterT w m a #

some :: WriterT w m a -> WriterT w m [a] #

many :: WriterT w m a -> WriterT w m [a] #

(Monoid w, MonadPlus m) => MonadPlus (WriterT w m) Source #

Instance details

Methods

mzero :: WriterT w m a #

mplus :: WriterT w m a -> WriterT w m a -> WriterT w m a #

(Eq w, Eq1 m, Eq a) => Eq (WriterT w m a) Source #

Instance details

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) Source #

Instance details

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) Source #

Instance details

Methods

readsPrec :: Int -> ReadS (WriterT w m a) #

readList :: ReadS [WriterT w m a] #

readPrec :: ReadPrec (WriterT w m a) #

readListPrec :: ReadPrec [WriterT w m a] #

(Show w, Show1 m, Show a) => Show (WriterT w m a) Source #

Instance details

Methods

showsPrec :: Int -> WriterT w m a -> ShowS #

show :: WriterT w m a -> String #

showList :: [WriterT w m a] -> ShowS #

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.