Safe Haskell  Safe 

Language  Haskell2010 
MSF
s with a Writer
monadic layer.
This module contains functions to work with MSF
s that include a Writer
monadic layer. This includes functions to create new MSF
s that include an
additional layer, and functions to flatten that layer out of the MSF
's
transformer stack.
It is based on the _strict_ writer monad Strict
,
so when combining it with other modules such as mtl
's,
the strict version has to be included, i.e. Strict
instead of Writer
or Lazy
.
Synopsis
 newtype WriterT w (m :: Type > Type) a = WriterT {
 runWriterT :: m (a, w)
 type Writer w = WriterT w Identity
 runWriter :: Writer w a > (a, w)
 execWriter :: Writer w a > w
 mapWriter :: ((a, w) > (b, w')) > Writer w a > Writer w' b
 execWriterT :: Monad m => WriterT w m a > m w
 mapWriterT :: (m (a, w) > n (b, w')) > WriterT w m a > WriterT w' n b
 censor :: Monad m => (w > w) > WriterT w m a > WriterT w m a
 listens :: Monad m => (w > b) > WriterT w m a > WriterT w m (a, b)
 listen :: Monad m => WriterT w m a > WriterT w m (a, w)
 tell :: Monad m => w > WriterT w m ()
 writer :: Monad m => (a, w) > WriterT w m a
 writerS :: (Functor m, Monad m, Monoid w) => MSF m a (w, b) > MSF (WriterT w m) a b
 runWriterS :: (Functor m, Monad m) => MSF (WriterT s m) a b > MSF m a (s, b)
Documentation
newtype WriterT w (m :: Type > Type) 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
.
WriterT  

Instances
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)
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)
censor :: Monad m => (w > w) > WriterT w m a > WriterT w m a #
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)
listens :: Monad m => (w > b) > WriterT w m a > WriterT w m (a, b) #
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)
listen :: Monad m => WriterT w m a > WriterT w m (a, w) #
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)
writer :: Monad m => (a, w) > WriterT w m a #
Construct a writer computation from a (result, output) pair.
(The inverse of runWriter
.)