| Copyright | (c) Andy Gill 2001, (c) Oregon Graduate Institute of Science and Technology, 2001 |
|---|---|
| License | BSD-style (see the file LICENSE) |
| Maintainer | R.Paterson@city.ac.uk |
| Stability | experimental |
| Portability | portable |
| Safe Haskell | Safe |
| Language | Haskell98 |
Control.Monad.Trans.Reader
Description
Declaration of the ReaderT monad transformer, which adds a static
environment to a given monad.
If the computation is to modify the stored information, use Control.Monad.Trans.State instead.
- type Reader r = ReaderT r Identity
- reader :: Monad m => (r -> a) -> ReaderT r m a
- runReader :: Reader r a -> r -> a
- mapReader :: (a -> b) -> Reader r a -> Reader r b
- withReader :: (r' -> r) -> Reader r a -> Reader r' a
- newtype ReaderT r m a = ReaderT {
- runReaderT :: r -> m a
- mapReaderT :: (m a -> n b) -> ReaderT r m a -> ReaderT r n b
- withReaderT :: (r' -> r) -> ReaderT r m a -> ReaderT r' m a
- ask :: Monad m => ReaderT r m r
- local :: (r -> r) -> ReaderT r m a -> ReaderT r m a
- asks :: Monad m => (r -> a) -> ReaderT r m a
- liftCallCC :: CallCC m a b -> CallCC (ReaderT r m) a b
- liftCatch :: Catch e m a -> Catch e (ReaderT r m) a
The Reader monad
type Reader r = ReaderT r Identity Source #
The parameterizable reader monad.
Computations are functions of a shared environment.
The return function ignores the environment, while >>= passes
the inherited environment to both subcomputations.
reader :: Monad m => (r -> a) -> ReaderT r m a Source #
Constructor for computations in the reader monad (equivalent to asks).
Arguments
| :: Reader r a | A |
| -> r | An initial environment. |
| -> a |
Runs a Reader and extracts the final value from it.
(The inverse of reader.)
Arguments
| :: (r' -> r) | The function to modify the environment. |
| -> Reader r a | Computation to run in the modified environment. |
| -> Reader r' a |
Execute a computation in a modified environment
(a specialization of withReaderT).
runReader(withReaderf m) =runReaderm . f
The ReaderT monad transformer
newtype ReaderT r m a Source #
The reader monad transformer, which adds a read-only environment to the given monad.
The return function ignores the environment, while >>= passes
the inherited environment to both subcomputations.
Constructors
| ReaderT | |
Fields
| |
Instances
| MonadTrans (ReaderT * r) # | |
| Monad m => Monad (ReaderT * r m) # | |
| Functor m => Functor (ReaderT * r m) # | |
| MonadFix m => MonadFix (ReaderT * r m) # | |
| MonadFail m => MonadFail (ReaderT * r m) # | |
| Applicative m => Applicative (ReaderT * r m) # | |
| MonadZip m => MonadZip (ReaderT * r m) # | |
| MonadIO m => MonadIO (ReaderT * r m) # | |
| Alternative m => Alternative (ReaderT * r m) # | |
| MonadPlus m => MonadPlus (ReaderT * r m) # | |
mapReaderT :: (m a -> n b) -> ReaderT r m a -> ReaderT r n b Source #
Transform the computation inside a ReaderT.
runReaderT(mapReaderTf m) = f .runReaderTm
Arguments
| :: (r' -> r) | The function to modify the environment. |
| -> ReaderT r m a | Computation to run in the modified environment. |
| -> ReaderT r' m a |
Execute a computation in a modified environment
(a more general version of local).
runReaderT(withReaderTf m) =runReaderTm . f
Reader operations
Arguments
| :: (r -> r) | The function to modify the environment. |
| -> ReaderT r m a | Computation to run in the modified environment. |
| -> ReaderT r m a |
Execute a computation in a modified environment
(a specialization of withReaderT).
runReaderT(localf m) =runReaderTm . f