Copyright	(c) Andy Gill 2001, (c) Oregon Graduate Institute of Science and Technology, 2001
License	BSD-style (see the file LICENSE)
Maintainer	ross@soi.city.ac.uk
Stability	experimental
Portability	portable
Safe Haskell	Safe-Inferred
Language	Haskell98

Control.Monad.Trans.Reader

Contents

The Reader monad
The ReaderT monad transformer
Reader operations
Lifting other operations

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.

Synopsis

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).

runReader Source

Arguments

:: Reader r a	A `Reader` to run.
-> 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.

runReader (mapReader f m) = f . runReader m

withReader Source

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 (withReader f m) = runReader m . 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 runReaderT :: r -> m a The underlying computation, as a function of the environment.

Instances

MonadTrans (ReaderT r)
Alternative m => Alternative (ReaderT r m)
Monad m => Monad (ReaderT r m)
Functor m => Functor (ReaderT r m)
MonadFix m => MonadFix (ReaderT r m)
MonadPlus m => MonadPlus (ReaderT r m)
Applicative m => Applicative (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 (mapReaderT f m) = f . runReaderT m

withReaderT Source

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 (withReaderT f m) = runReaderT m . f

Reader operations

ask :: Monad m => ReaderT r m r Source

Fetch the value of the environment.

local Source

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 (local f m) = runReaderT m . f

asks Source

Arguments

:: Monad m
=> (r -> a)	The selector function to apply to the environment.
-> ReaderT r m a

Retrieve a function of the current environment.

```
asks f = liftM f ask
```

Lifting other operations

liftCallCC Source

Arguments

:: (((a -> m b) -> m a) -> m a)	`callCC` on the argument monad.
-> ((a -> ReaderT r m b) -> ReaderT r m a)
-> ReaderT r m a

Lift a callCC operation to the new monad.

liftCatch Source

Arguments

:: (m a -> (e -> m a) -> m a)	`catch` on the argument monad.
-> ReaderT r m a	Computation to attempt.
-> (e -> ReaderT r m a)	Exception handler.
-> ReaderT r m a

Lift a catchError operation to the new monad.