Control.Monad.Writer.Lazy

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.

MonadWriter class

class (Monoid (WriterType m), Monad m) => MonadWriter m where

Minimal complete definition

tell, listen, pass

Associated Types

type WriterType m

Methods

tell :: WriterType m -> m ()

listen :: m a -> m (a, WriterType m)

pass :: m (a, WriterType m -> WriterType m) -> m a

Instances

MonadWriter m => MonadWriter (MaybeT m)

Associated Types

type WriterType (MaybeT m :: * -> *) :: *

Methods

tell :: WriterType (MaybeT m) -> MaybeT m ()

listen :: MaybeT m a -> MaybeT m (a, WriterType (MaybeT m))

pass :: MaybeT m (a, WriterType (MaybeT m) -> WriterType (MaybeT m)) -> MaybeT m a

(Monoid w, Monad m) => MonadWriter (WriterT w m)

Associated Types

type WriterType (WriterT w m :: * -> *) :: *

Methods

tell :: WriterType (WriterT w m) -> WriterT w m ()

listen :: WriterT w m a -> WriterT w m (a, WriterType (WriterT w m))

pass :: WriterT w m (a, WriterType (WriterT w m) -> WriterType (WriterT w m)) -> WriterT w m a

(Monoid w, Monad m) => MonadWriter (WriterT w m)

Associated Types

type WriterType (WriterT w m :: * -> *) :: *

Methods

tell :: WriterType (WriterT w m) -> WriterT w m ()

listen :: WriterT w m a -> WriterT w m (a, WriterType (WriterT w m))

pass :: WriterT w m (a, WriterType (WriterT w m) -> WriterType (WriterT w m)) -> WriterT w m a

MonadWriter m => MonadWriter (StateT s m)

Associated Types

type WriterType (StateT s m :: * -> *) :: *

Methods

tell :: WriterType (StateT s m) -> StateT s m ()

listen :: StateT s m a -> StateT s m (a, WriterType (StateT s m))

pass :: StateT s m (a, WriterType (StateT s m) -> WriterType (StateT s m)) -> StateT s m a

MonadWriter m => MonadWriter (StateT s m)

Associated Types

type WriterType (StateT s m :: * -> *) :: *

Methods

tell :: WriterType (StateT s m) -> StateT s m ()

listen :: StateT s m a -> StateT s m (a, WriterType (StateT s m))

pass :: StateT s m (a, WriterType (StateT s m) -> WriterType (StateT s m)) -> StateT s m a

MonadWriter m => MonadWriter (IdentityT * m)

Associated Types

type WriterType (IdentityT * m :: * -> *) :: *

Methods

tell :: WriterType (IdentityT * m) -> IdentityT * m ()

listen :: IdentityT * m a -> IdentityT * m (a, WriterType (IdentityT * m))

pass :: IdentityT * m (a, WriterType (IdentityT * m) -> WriterType (IdentityT * m)) -> IdentityT * m a

(Error e, MonadWriter m) => MonadWriter (ErrorT e m)

Associated Types

type WriterType (ErrorT e m :: * -> *) :: *

Methods

tell :: WriterType (ErrorT e m) -> ErrorT e m ()

listen :: ErrorT e m a -> ErrorT e m (a, WriterType (ErrorT e m))

pass :: ErrorT e m (a, WriterType (ErrorT e m) -> WriterType (ErrorT e m)) -> ErrorT e m a

MonadWriter m => MonadWriter (ReaderT * r m)

Associated Types

type WriterType (ReaderT * r m :: * -> *) :: *

Methods

tell :: WriterType (ReaderT * r m) -> ReaderT * r m ()

listen :: ReaderT * r m a -> ReaderT * r m (a, WriterType (ReaderT * r m))

pass :: ReaderT * r m (a, WriterType (ReaderT * r m) -> WriterType (ReaderT * r m)) -> ReaderT * r m a

(Monoid w, Monad m) => MonadWriter (RWST r w s m)

Associated Types

type WriterType (RWST r w s m :: * -> *) :: *

Methods

tell :: WriterType (RWST r w s m) -> RWST r w s m ()

listen :: RWST r w s m a -> RWST r w s m (a, WriterType (RWST r w s m))

pass :: RWST r w s m (a, WriterType (RWST r w s m) -> WriterType (RWST r w s m)) -> RWST r w s m a

(Monoid w, Monad m) => MonadWriter (RWST r w s m)

Associated Types

type WriterType (RWST r w s m :: * -> *) :: *

Methods

tell :: WriterType (RWST r w s m) -> RWST r w s m ()

listen :: RWST r w s m a -> RWST r w s m (a, WriterType (RWST r w s m))

pass :: RWST r w s m (a, WriterType (RWST r w s m) -> WriterType (RWST r w s m)) -> RWST r w s m a

listens :: MonadWriter m => (WriterType m -> b) -> m a -> m (a, b)

censor :: MonadWriter m => (WriterType m -> WriterType m) -> m a -> m a

The Writer monad

type Writer w = WriterT w Identity

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.

runWriter :: Writer w a -> (a, w)

Unwrap a writer computation as a (result, output) pair. (The inverse of writer.)

execWriter :: Writer w a -> w

Extract the output from a writer computation.

• execWriter m = snd (runWriter m)

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

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 :: * -> (* -> *) -> * -> *

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 => MonadTrans (WriterT w)

Methods

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

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

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)

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)

Methods

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

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

Methods

fail :: String -> WriterT w m a

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

Methods

pure :: a -> WriterT w m a

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

(*>) :: 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)

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)

Methods

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

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

Methods

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

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

Methods

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

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

Methods

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

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

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

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)

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)

Methods

liftIO :: IO a -> WriterT w m a

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

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)

Methods

mzero :: WriterT w m a

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

(Monoid w, MonadCont m) => MonadCont (WriterT w m)

Methods

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

(Monoid w, MonadError m) => MonadError (WriterT w m)

Associated Types

type ErrorType (WriterT w m :: * -> *) :: *

Methods

throwError :: ErrorType (WriterT w m) -> WriterT w m a

catchError :: WriterT w m a -> (ErrorType (WriterT w m) -> WriterT w m a) -> WriterT w m a

(Monoid w, MonadReader m) => MonadReader (WriterT w m)

Associated Types

type EnvType (WriterT w m :: * -> *) :: *

Methods

ask :: WriterT w m (EnvType (WriterT w m))

local :: (EnvType (WriterT w m) -> EnvType (WriterT w m)) -> WriterT w m a -> WriterT w m a

(Monoid w, MonadState m) => MonadState (WriterT w m)

Associated Types

type StateType (WriterT w m :: * -> *) :: *

Methods

get :: WriterT w m (StateType (WriterT w m))

put :: StateType (WriterT w m) -> WriterT w m ()

(Monoid w, Monad m) => MonadWriter (WriterT w m)

Associated Types

type WriterType (WriterT w m :: * -> *) :: *

Methods

tell :: WriterType (WriterT w m) -> WriterT w m ()

listen :: WriterT w m a -> WriterT w m (a, WriterType (WriterT w m))

pass :: WriterT w m (a, WriterType (WriterT w m) -> WriterType (WriterT w m)) -> WriterT w m a

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

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)

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)

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)

Methods

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

show :: WriterT w m a -> String

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

type ErrorType (WriterT w m)

type ErrorType (WriterT w m) = ErrorType m

type EnvType (WriterT w m)

type EnvType (WriterT w m) = EnvType m

type StateType (WriterT w m)

type StateType (WriterT w m) = StateType m

type WriterType (WriterT w m)

type WriterType (WriterT w m) = w

execWriterT :: Monad m => WriterT w m a -> m w

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

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

• runWriterT (mapWriterT f m) = f (runWriterT m)

module Control.Monad

module Control.Monad.Fix

module Control.Monad.Trans

module Data.Monoid