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Control.Monad.Reader.Class

Copyright (c) Andy Gill 2001
(c) Oregon Graduate Institute of Science and Technology 2001
(c) Jeff Newbern 2003-2007
(c) Andriy Palamarchuk 2007
License BSD-style (see the file LICENSE)
Maintainer [email protected]
Stability experimental
Portability non-portable (multi-param classes, functional dependencies)
Safe Haskell Safe
Language Haskell2010

Description

Computation type:
Computations which read values from a shared environment.
Binding strategy:
Monad values are functions from the environment to a value. The bound function is applied to the bound value, and both have access to the shared environment.
Useful for:
Maintaining variable bindings, or other shared environment.
Zero and plus:
None.
Example type:
Reader [(String,Value)] a

The Reader monad (also called the Environment monad). Represents a computation, which can read values from a shared environment, pass values from function to function, and execute sub-computations in a modified environment. Using Reader monad for such computations is often clearer and easier than using the State monad.

Inspired by the paper Functional Programming with Overloading and Higher-Order Polymorphism, Mark P Jones (http://web.cecs.pdx.edu/~mpj/) Advanced School of Functional Programming, 1995.

class Monad m => MonadReader r m | m -> r where Source

See examples in Control.Monad.Reader. Note, the partially applied function type (->) r is a simple reader monad. See the instance declaration below.

Minimal complete definition

(ask | reader), local

Methods

ask :: m r Source

Retrieves the monad environment.

local Source

Arguments

:: (r -> r)

The function to modify the environment.

-> m a

Reader to run in the modified environment.

-> m a

Executes a computation in a modified environment.

reader Source

Arguments

:: (r -> a)

The selector function to apply to the environment.

-> m a

Retrieves a function of the current environment.

Instances
MonadReader r m => MonadReader r (MaybeT m)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: MaybeT m r Source

local :: (r -> r) -> MaybeT m a -> MaybeT m a Source

reader :: (r -> a) -> MaybeT m a Source

MonadReader r m => MonadReader r (ListT m)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: ListT m r Source

local :: (r -> r) -> ListT m a -> ListT m a Source

reader :: (r -> a) -> ListT m a Source

(Monoid w, MonadReader r m) => MonadReader r (WriterT w m)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: WriterT w m r Source

local :: (r -> r) -> WriterT w m a -> WriterT w m a Source

reader :: (r -> a) -> WriterT w m a Source

(Monoid w, MonadReader r m) => MonadReader r (WriterT w m)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: WriterT w m r Source

local :: (r -> r) -> WriterT w m a -> WriterT w m a Source

reader :: (r -> a) -> WriterT w m a Source

MonadReader r m => MonadReader r (StateT s m)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: StateT s m r Source

local :: (r -> r) -> StateT s m a -> StateT s m a Source

reader :: (r -> a) -> StateT s m a Source

MonadReader r m => MonadReader r (StateT s m)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: StateT s m r Source

local :: (r -> r) -> StateT s m a -> StateT s m a Source

reader :: (r -> a) -> StateT s m a Source

MonadReader r m => MonadReader r (IdentityT m)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: IdentityT m r Source

local :: (r -> r) -> IdentityT m a -> IdentityT m a Source

reader :: (r -> a) -> IdentityT m a Source

MonadReader r m => MonadReader r (ExceptT e m)

Since: mtl-2.2

Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: ExceptT e m r Source

local :: (r -> r) -> ExceptT e m a -> ExceptT e m a Source

reader :: (r -> a) -> ExceptT e m a Source

(Error e, MonadReader r m) => MonadReader r (ErrorT e m)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: ErrorT e m r Source

local :: (r -> r) -> ErrorT e m a -> ErrorT e m a Source

reader :: (r -> a) -> ErrorT e m a Source

MonadReader r' m => MonadReader r' (ContT r m)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: ContT r m r' Source

local :: (r' -> r') -> ContT r m a -> ContT r m a Source

reader :: (r' -> a) -> ContT r m a Source

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

Defined in Control.Monad.Reader.Class

Methods

ask :: ReaderT r m r Source

local :: (r -> r) -> ReaderT r m a -> ReaderT r m a Source

reader :: (r -> a) -> ReaderT r m a Source

MonadReader r ((->) r :: Type -> Type)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: r -> r Source

local :: (r -> r) -> (r -> a) -> r -> a Source

reader :: (r -> a) -> r -> a Source

(Monad m, Monoid w) => MonadReader r (RWST r w s m)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: RWST r w s m r Source

local :: (r -> r) -> RWST r w s m a -> RWST r w s m a Source

reader :: (r -> a) -> RWST r w s m a Source

(Monad m, Monoid w) => MonadReader r (RWST r w s m)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: RWST r w s m r Source

local :: (r -> r) -> RWST r w s m a -> RWST r w s m a Source

reader :: (r -> a) -> RWST r w s m a Source

asks Source

Arguments

:: MonadReader r m
=> (r -> a)

The selector function to apply to the environment.

-> m a

Retrieves a function of the current environment.

© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/8.6.1/docs/html/libraries/mtl-2.2.2/Control-Monad-Reader-Class.html