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

Instances

MonadTrans (ReaderT * r)

Methods

lift :: Monad m => m a -> ReaderT * r m a

Monad m => Monad (ReaderT * r m)

Methods

(>>=) :: ReaderT * r m a -> (a -> ReaderT * r m b) -> ReaderT * r m b

(>>) :: ReaderT * r m a -> ReaderT * r m b -> ReaderT * r m b

return :: a -> ReaderT * r m a

fail :: String -> ReaderT * r m a

Functor m => Functor (ReaderT * r m)

Methods

fmap :: (a -> b) -> ReaderT * r m a -> ReaderT * r m b

(<$) :: a -> ReaderT * r m b -> ReaderT * r m a

MonadFix m => MonadFix (ReaderT * r m)

Methods

mfix :: (a -> ReaderT * r m a) -> ReaderT * r m a

MonadFail m => MonadFail (ReaderT * r m)

Methods

fail :: String -> ReaderT * r m a

Applicative m => Applicative (ReaderT * r m)

Methods

pure :: a -> ReaderT * r m a

(<*>) :: ReaderT * r m (a -> b) -> ReaderT * r m a -> ReaderT * r m b

liftA2 :: (a -> b -> c) -> ReaderT * r m a -> ReaderT * r m b -> ReaderT * r m c

(*>) :: ReaderT * r m a -> ReaderT * r m b -> ReaderT * r m b

(<*) :: ReaderT * r m a -> ReaderT * r m b -> ReaderT * r m a

MonadZip m => MonadZip (ReaderT * r m)

Methods

mzip :: ReaderT * r m a -> ReaderT * r m b -> ReaderT * r m (a, b)

mzipWith :: (a -> b -> c) -> ReaderT * r m a -> ReaderT * r m b -> ReaderT * r m c

munzip :: ReaderT * r m (a, b) -> (ReaderT * r m a, ReaderT * r m b)

MonadIO m => MonadIO (ReaderT * r m)

Methods

liftIO :: IO a -> ReaderT * r m a

Alternative m => Alternative (ReaderT * r m)

Methods

empty :: ReaderT * r m a

(<|>) :: ReaderT * r m a -> ReaderT * r m a -> ReaderT * r m a

some :: ReaderT * r m a -> ReaderT * r m [a]

many :: ReaderT * r m a -> ReaderT * r m [a]

MonadPlus m => MonadPlus (ReaderT * r m)

Methods

mzero :: ReaderT * r m a

mplus :: ReaderT * r m a -> ReaderT * r m a -> ReaderT * r m a

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

:: (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 :: CallCC m a b -> CallCC (ReaderT r m) a b

Lift a callCC operation to the new monad.

liftCatch :: Catch e m a -> Catch e (ReaderT r m) a

Lift a catchE operation to the new monad.