transformers-0.5.5.0: Concrete functor and monad transformers

Copyright(c) Andy Gill 2001
(c) Oregon Graduate Institute of Science and Technology 2001
LicenseBSD-style (see the file LICENSE)
MaintainerR.Paterson@city.ac.uk
Stabilityexperimental
Portabilityportable
Safe HaskellSafe
LanguageHaskell98

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 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.

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).

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

Instances

MonadTrans (ReaderT * r) Source # 

Methods

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

Monad m => Monad (ReaderT * r m) Source # 

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) Source # 

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) Source # 

Methods

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

MonadFail m => MonadFail (ReaderT * r m) Source # 

Methods

fail :: String -> ReaderT * r m a #

Applicative m => Applicative (ReaderT * r m) Source # 

Methods

pure :: a -> ReaderT * r m a #

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

(*>) :: 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) Source # 

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) Source # 

Methods

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

Alternative m => Alternative (ReaderT * r m) Source # 

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) Source # 

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 Source #

Transform the computation inside a ReaderT.

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).

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).

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.

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.