Control.Monad.Trans.Reader
| 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 |
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.
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.)
mapReader :: (a -> b) -> Reader r a -> Reader r b Source
Transform the value returned by a 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
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) | |
| Applicative m => Applicative (ReaderT r m) | |
| Alternative m => Alternative (ReaderT r m) | |
| MonadPlus m => MonadPlus (ReaderT r m) | |
| MonadIO m => MonadIO (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
ask :: Monad m => ReaderT r m r Source
Fetch the value of the environment.
Arguments
| :: Monad m | |
| => (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
Arguments
| :: Monad m | |
| => (r -> a) | The selector function to apply to the environment. |
| -> ReaderT r m a |
Retrieve a function of the current environment.
Lifting other operations
liftCallCC :: CallCC m a b -> CallCC (ReaderT r m) a b Source
Lift a callCC operation to the new monad.
liftCatch :: Catch e m a -> Catch e (ReaderT r 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/7.10.3/docs/html/libraries/transformers-0.4.2.0/Control-Monad-Trans-Reader.html