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

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

Instances

MonadTrans (ReaderT (TYPE Lifted) r)
Methods lift :: Monad m => m a -> ReaderT (TYPE Lifted) r m a Source
Monad m => Monad (ReaderT (TYPE Lifted) r m)
Methods (>>=) :: ReaderT (TYPE Lifted) r m a -> (a -> ReaderT (TYPE Lifted) r m b) -> ReaderT (TYPE Lifted) r m b Source (>>) :: ReaderT (TYPE Lifted) r m a -> ReaderT (TYPE Lifted) r m b -> ReaderT (TYPE Lifted) r m b Source return :: a -> ReaderT (TYPE Lifted) r m a Source fail :: String -> ReaderT (TYPE Lifted) r m a Source
Functor m => Functor (ReaderT (TYPE Lifted) r m)
Methods fmap :: (a -> b) -> ReaderT (TYPE Lifted) r m a -> ReaderT (TYPE Lifted) r m b Source (<$) :: a -> ReaderT (TYPE Lifted) r m b -> ReaderT (TYPE Lifted) r m a Source
MonadFix m => MonadFix (ReaderT (TYPE Lifted) r m)
Methods mfix :: (a -> ReaderT (TYPE Lifted) r m a) -> ReaderT (TYPE Lifted) r m a Source
MonadFail m => MonadFail (ReaderT (TYPE Lifted) r m)
Methods fail :: String -> ReaderT (TYPE Lifted) r m a Source
Applicative m => Applicative (ReaderT (TYPE Lifted) r m)
Methods pure :: a -> ReaderT (TYPE Lifted) r m a Source (<>) :: ReaderT (TYPE Lifted) r m (a -> b) -> ReaderT (TYPE Lifted) r m a -> ReaderT (TYPE Lifted) r m b Source (>) :: ReaderT (TYPE Lifted) r m a -> ReaderT (TYPE Lifted) r m b -> ReaderT (TYPE Lifted) r m b Source (<*) :: ReaderT (TYPE Lifted) r m a -> ReaderT (TYPE Lifted) r m b -> ReaderT (TYPE Lifted) r m a Source
MonadIO m => MonadIO (ReaderT (TYPE Lifted) r m)
Methods liftIO :: IO a -> ReaderT (TYPE Lifted) r m a Source
MonadZip m => MonadZip (ReaderT (TYPE Lifted) r m)
Methods mzip :: ReaderT (TYPE Lifted) r m a -> ReaderT (TYPE Lifted) r m b -> ReaderT (TYPE Lifted) r m (a, b) Source mzipWith :: (a -> b -> c) -> ReaderT (TYPE Lifted) r m a -> ReaderT (TYPE Lifted) r m b -> ReaderT (TYPE Lifted) r m c Source munzip :: ReaderT (TYPE Lifted) r m (a, b) -> (ReaderT (TYPE Lifted) r m a, ReaderT (TYPE Lifted) r m b) Source
Alternative m => Alternative (ReaderT (TYPE Lifted) r m)
Methods empty :: ReaderT (TYPE Lifted) r m a Source (<\|>) :: ReaderT (TYPE Lifted) r m a -> ReaderT (TYPE Lifted) r m a -> ReaderT (TYPE Lifted) r m a Source some :: ReaderT (TYPE Lifted) r m a -> ReaderT (TYPE Lifted) r m [a] Source many :: ReaderT (TYPE Lifted) r m a -> ReaderT (TYPE Lifted) r m [a] Source
MonadPlus m => MonadPlus (ReaderT (TYPE Lifted) r m)
Methods mzero :: ReaderT (TYPE Lifted) r m a Source mplus :: ReaderT (TYPE Lifted) r m a -> ReaderT (TYPE Lifted) r m a -> ReaderT (TYPE Lifted) r m a Source

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

:: (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 :: 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.