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/Haskell 9

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 Haskell2010

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 m >>= k passes the inherited environment to both subcomputations:

reader :: forall (m :: Type -> Type) r a. 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 :: Type -> Type) a Source

The reader monad transformer, which adds a read-only environment to the given monad.

The return function ignores the environment, while m >>= k passes the inherited environment to both subcomputations:

Constructors

ReaderT

Fields

Instances
Instances details
Generic1 (ReaderT r m :: Type -> Type) Source
Instance details

Defined in Control.Monad.Trans.Reader

Associated Types

type Rep1 (ReaderT r m :: Type -> Type)
Instance details

Defined in Control.Monad.Trans.Reader

type Rep1 (ReaderT r m :: Type -> Type) = D1 ('MetaData "ReaderT" "Control.Monad.Trans.Reader" "transformers-0.6.1.2-72bd" 'True) (C1 ('MetaCons "ReaderT" 'PrefixI 'True) (S1 ('MetaSel ('Just "runReaderT") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) ((FUN 'Many r :: Type -> Type) :.: Rec1 m)))

Methods

from1 :: ReaderT r m a -> Rep1 (ReaderT r m) a

to1 :: Rep1 (ReaderT r m) a -> ReaderT r m a

MonadTrans (ReaderT r) Source
Instance details

Defined in Control.Monad.Trans.Reader

Methods

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

Contravariant m => Contravariant (ReaderT r m) Source
Instance details

Defined in Control.Monad.Trans.Reader

Methods

contramap :: (a' -> a) -> ReaderT r m a -> ReaderT r m a' Source

(>$) :: b -> ReaderT r m b -> ReaderT r m a Source

Alternative m => Alternative (ReaderT r m) Source
Instance details

Defined in Control.Monad.Trans.Reader

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]

Applicative m => Applicative (ReaderT r m) Source
Instance details

Defined in Control.Monad.Trans.Reader

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

Functor m => Functor (ReaderT r m) Source
Instance details

Defined in Control.Monad.Trans.Reader

Methods

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

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

Monad m => Monad (ReaderT r m) Source
Instance details

Defined in Control.Monad.Trans.Reader

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

MonadPlus m => MonadPlus (ReaderT r m) Source
Instance details

Defined in Control.Monad.Trans.Reader

Methods

mzero :: ReaderT r m a

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

MonadFail m => MonadFail (ReaderT r m) Source
Instance details

Defined in Control.Monad.Trans.Reader

Methods

fail :: String -> ReaderT r m a

MonadFix m => MonadFix (ReaderT r m) Source
Instance details

Defined in Control.Monad.Trans.Reader

Methods

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

MonadIO m => MonadIO (ReaderT r m) Source
Instance details

Defined in Control.Monad.Trans.Reader

Methods

liftIO :: IO a -> ReaderT r m a

MonadZip m => MonadZip (ReaderT r m) Source
Instance details

Defined in Control.Monad.Trans.Reader

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)

Generic (ReaderT r m a) Source
Instance details

Defined in Control.Monad.Trans.Reader

Associated Types

type Rep (ReaderT r m a)
Instance details

Defined in Control.Monad.Trans.Reader

type Rep (ReaderT r m a) = D1 ('MetaData "ReaderT" "Control.Monad.Trans.Reader" "transformers-0.6.1.2-72bd" 'True) (C1 ('MetaCons "ReaderT" 'PrefixI 'True) (S1 ('MetaSel ('Just "runReaderT") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 (r -> m a))))

Methods

from :: ReaderT r m a -> Rep (ReaderT r m a) x

to :: Rep (ReaderT r m a) x -> ReaderT r m a

type Rep1 (ReaderT r m :: Type -> Type) Source
Instance details

Defined in Control.Monad.Trans.Reader

type Rep1 (ReaderT r m :: Type -> Type) = D1 ('MetaData "ReaderT" "Control.Monad.Trans.Reader" "transformers-0.6.1.2-72bd" 'True) (C1 ('MetaCons "ReaderT" 'PrefixI 'True) (S1 ('MetaSel ('Just "runReaderT") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) ((FUN 'Many r :: Type -> Type) :.: Rec1 m)))
type Rep (ReaderT r m a) Source
Instance details

Defined in Control.Monad.Trans.Reader

type Rep (ReaderT r m a) = D1 ('MetaData "ReaderT" "Control.Monad.Trans.Reader" "transformers-0.6.1.2-72bd" 'True) (C1 ('MetaCons "ReaderT" 'PrefixI 'True) (S1 ('MetaSel ('Just "runReaderT") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 (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

:: forall r' r (m :: Type -> Type) a. (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 :: forall (m :: Type -> Type) r. Monad m => ReaderT r m r Source

Fetch the value of the environment.

local Source

Arguments

:: forall r (m :: Type -> Type) a. (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

:: forall (m :: Type -> Type) r a. 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/9.12.1/docs/libraries/transformers-0.6.1.2-72bd/Control-Monad-Trans-Reader.html