Copyright | (c) Andy Gill 2001 (c) Oregon Graduate Institute of Science and Technology 2001 |
---|---|
License | BSD-style (see the file LICENSE) |
Maintainer | libraries@haskell.org |
Stability | experimental |
Portability | non-portable (multi-param classes, functional dependencies) |
Safe Haskell | Safe |
Language | Haskell2010 |
The MonadWriter class.
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.
Synopsis
- class (Monoid w, Monad m) => MonadWriter w m | m -> w where
- listens :: MonadWriter w m => (w -> b) -> m a -> m (a, b)
- censor :: MonadWriter w m => (w -> w) -> m a -> m a
Documentation
class (Monoid w, Monad m) => MonadWriter w m | m -> w where Source #
writer :: (a, w) -> m a Source #
embeds a simple writer action.writer
(a,w)
is an action that produces the output tell
ww
.
listen :: m a -> m (a, w) Source #
is an action that executes the action listen
mm
and adds
its output to the value of the computation.
pass :: m (a, w -> 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.
Instances
MonadWriter w m => MonadWriter w (MaybeT m) # | |
Monoid w => MonadWriter w ((,) w) # | NOTE: This instance is only defined for Since: mtl-2.2.2 |
MonadWriter w m => MonadWriter w (StateT s m) # | |
MonadWriter w m => MonadWriter w (StateT s m) # | |
MonadWriter w m => MonadWriter w (IdentityT m) # | |
MonadWriter w m => MonadWriter w (ExceptT e m) # | Since: mtl-2.2 |
(Error e, MonadWriter w m) => MonadWriter w (ErrorT e m) # | |
(Monoid w, Monad m) => MonadWriter w (WriterT w m) # | |
(Monoid w, Monad m) => MonadWriter w (WriterT w m) # | |
MonadWriter w m => MonadWriter w (ReaderT r m) # | |
(Monoid w, Monad m) => MonadWriter w (RWST r w s m) # | |
(Monoid w, Monad m) => MonadWriter w (RWST r w s m) # | |
listens :: MonadWriter w m => (w -> b) -> m a -> m (a, b) Source #