transformers-0.2.2.0: Concrete functor and monad transformers

Portabilityportable
Stabilityexperimental
Maintainerross@soi.city.ac.uk

Control.Monad.Trans.Reader

Contents

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 IdentitySource

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 :: (r -> a) -> Reader r aSource

Constructor for computations in the reader monad.

runReaderSource

Arguments

:: Reader r a

A Reader to run.

-> r

An initial environment.

-> a 

Runs a Reader and extracts the final value from it.

mapReader :: (a -> b) -> Reader r a -> Reader r bSource

Transform the value returned by a Reader.

withReaderSource

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).

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.

mapReaderT :: (m a -> n b) -> ReaderT r m a -> ReaderT r n bSource

Transform the computation inside a ReaderT.

withReaderTSource

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).

Reader operations

ask :: Monad m => ReaderT r m rSource

Fetch the value of the environment.

localSource

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).

asksSource

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

liftCallCCSource

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.

liftCatchSource

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.