| Copyright | (c) Andy Gill 2001 (c) Oregon Graduate Institute of Science and Technology 2001 |
|---|---|
| License | BSD-style (see the file LICENSE) |
| Maintainer | [email protected] |
| Stability | experimental |
| Portability | portable |
| Safe Haskell | Safe |
| Language | Haskell98 |
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.Writer.CPS instead.
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.
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 | |
Fields
| |
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)
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)
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.
© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/8.8.3/docs/html/libraries/transformers-0.5.6.2/Control-Monad-Trans-Writer-Strict.html