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Description | ||||||||||||||||||||||||||||||
The lazy WriterT monad transformer, which adds collection of outputs (such as a count or string output) to a given monad. This version builds its output lazily; for a strict version, see Control.Monad.Trans.Writer.Strict, which has the same interface. This monad transformer provides only limited access to the output during the computation. For more general access, use Control.Monad.Trans.State instead. | ||||||||||||||||||||||||||||||
Synopsis | ||||||||||||||||||||||||||||||
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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. | ||||||||||||||||||||||||||||||
writer :: (a, w) -> Writer w a | ||||||||||||||||||||||||||||||
Construct a writer computation from a (result, output) pair. (The inverse of runWriter.) | ||||||||||||||||||||||||||||||
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.
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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. | ||||||||||||||||||||||||||||||
The WriterT monad transformer | ||||||||||||||||||||||||||||||
newtype WriterT w m a | ||||||||||||||||||||||||||||||
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execWriterT :: Monad m => WriterT w m a -> m w | ||||||||||||||||||||||||||||||
Extract the output from a writer computation.
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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.
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Writer operations | ||||||||||||||||||||||||||||||
tell :: (Monoid w, Monad m) => w -> WriterT w m () | ||||||||||||||||||||||||||||||
tell w is an action that produces the output w. | ||||||||||||||||||||||||||||||
listen :: (Monoid w, Monad m) => WriterT w m a -> WriterT w m (a, w) | ||||||||||||||||||||||||||||||
listen m is an action that executes the action m and adds its output to the value of the computation.
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listens :: (Monoid w, Monad m) => (w -> b) -> WriterT w m a -> WriterT w m (a, b) | ||||||||||||||||||||||||||||||
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.
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pass :: (Monoid w, Monad m) => WriterT w m (a, w -> w) -> WriterT w m a | ||||||||||||||||||||||||||||||
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.
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censor :: (Monoid w, Monad m) => (w -> w) -> WriterT w m a -> WriterT w m a | ||||||||||||||||||||||||||||||
censor f m is an action that executes the action m and applies the function f to its output, leaving the return value unchanged.
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Lifting other operations | ||||||||||||||||||||||||||||||
liftCallCC :: Monoid w => ((((a, w) -> m (b, w)) -> m (a, w)) -> m (a, w)) -> ((a -> WriterT w m b) -> WriterT w m a) -> WriterT w m a | ||||||||||||||||||||||||||||||
Lift a callCC operation to the new monad. | ||||||||||||||||||||||||||||||
liftCatch :: (m (a, w) -> (e -> m (a, w)) -> m (a, w)) -> WriterT w m a -> (e -> WriterT w m a) -> WriterT w m a | ||||||||||||||||||||||||||||||
Lift a catchError operation to the new monad. | ||||||||||||||||||||||||||||||
Produced by Haddock version 2.7.2 |