Control.Monad.Trans.Reader

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

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

Constructor for computations in the reader monad (equivalent to asks).

runReader

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

Transform the value returned by a Reader.

• runReader (mapReader f m) = f . runReader m

withReader

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

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)

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)

Alternative m => Alternative (ReaderT r m)

MonadIO m => MonadIO (ReaderT r m)

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

Transform the computation inside a ReaderT.

• runReaderT (mapReaderT f m) = f . runReaderT m

withReaderT

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

Fetch the value of the environment.

local

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

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

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

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.