Control.Monad.Trans.Reader

Copyright (c) Andy Gill 2001, (c) Oregon Graduate Institute of Science and Technology, 2001
License BSD-style (see the file LICENSE)
Maintainer [email protected]
Stability experimental
Portability portable
Safe Haskell Safe
Language Haskell98

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.

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

runReaderT :: 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

:: 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).

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.

© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/7.10.3/docs/html/libraries/transformers-0.4.2.0/Control-Monad-Trans-Reader.html