Safe Haskell | Safe |
---|---|
Language | Haskell2010 |
- type Writer w = WriterT w Identity
- writer :: (Monoid w, Monad m) => (a, w) -> WriterT w m a
- runWriter :: Monoid w => Writer w a -> (a, w)
- execWriter :: Monoid w => Writer w a -> w
- mapWriter :: (Monoid w, Monoid w') => ((a, w) -> (b, w')) -> Writer w a -> Writer w' b
- data WriterT w m a
- runWriterT :: Monoid w => WriterT w m a -> m (a, w)
- execWriterT :: (Monad m, Monoid w) => WriterT w m a -> m w
- mapWriterT :: (Monad n, Monoid w, Monoid w') => (m (a, w) -> n (b, w')) -> WriterT w m a -> WriterT w' n b
- tell :: (Monoid w, Monad m) => w -> WriterT w m ()
- listen :: (Monoid w, Monad m) => WriterT w m a -> WriterT w m (a, w)
- listens :: (Monoid w, Monad m) => (w -> b) -> WriterT w m a -> WriterT w m (a, b)
- pass :: (Monoid w, Monoid w', Monad m) => WriterT w m (a, w -> w') -> WriterT w' m a
- censor :: (Monoid w, Monad m) => (w -> w) -> WriterT w m a -> WriterT w m a
The Writer monad
writer :: (Monoid w, Monad m) => (a, w) -> WriterT w m a Source #
Construct a writer computation from a (result, output) pair.
(The inverse of runWriter
.)
runWriter :: Monoid w => Writer w a -> (a, w) Source #
Unwrap a writer computation as a (result, output) pair.
(The inverse of writer
.)
execWriter :: Monoid w => Writer w a -> w Source #
Extract the output from a writer computation.
execWriter
m =snd
(runWriter
m)
The WriterT monad transformer
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
.
MonadTrans (WriterT w) Source # | |
Monad m => Monad (WriterT w m) Source # | |
Functor m => Functor (WriterT w m) Source # | |
MonadFix m => MonadFix (WriterT w m) Source # | |
MonadFail m => MonadFail (WriterT w m) Source # | |
(Functor m, Monad m) => Applicative (WriterT w m) Source # | |
MonadIO m => MonadIO (WriterT w m) Source # | |
(Functor m, MonadPlus m) => Alternative (WriterT w m) Source # | |
(Functor m, MonadPlus m) => MonadPlus (WriterT w m) Source # | |
runWriterT :: Monoid w => WriterT w m a -> m (a, w) Source #
Unwrap a writer computation.
execWriterT :: (Monad m, Monoid w) => WriterT w m a -> m w Source #
Extract the output from a writer computation.
execWriterT
m =liftM
snd
(runWriterT
m)
mapWriterT :: (Monad n, Monoid w, Monoid w') => (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 :: (Monoid w, Monad m) => w -> WriterT w m () Source #
is an action that produces the output tell
ww
.
listen :: (Monoid w, Monad m) => WriterT w m a -> WriterT w m (a, w) Source #
is an action that executes the action listen
mm
and adds its
output to the value of the computation.
runWriterT
(listen
m) =liftM
(\ (a, w) -> ((a, w), w)) (runWriterT
m)
listens :: (Monoid w, Monad m) => (w -> b) -> WriterT w m a -> WriterT w m (a, b) Source #
is an action that executes the action listens
f mm
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 :: (Monoid w, Monoid w', Monad m) => WriterT w m (a, w -> w') -> WriterT 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.
runWriterT
(pass
m) =liftM
(\ ((a, f), w) -> (a, f w)) (runWriterT
m)
censor :: (Monoid w, Monad m) => (w -> w) -> WriterT w m a -> WriterT w m a Source #
is an action that executes the action censor
f mm
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)