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	libraries@haskell.org
Stability	experimental
Portability	non-portable (multi-param classes, functional dependencies)
Safe Haskell	Safe
Language	Haskell2010

Control.Monad.Reader.Class

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.

Synopsis

class Monad m => MonadReader r m | m -> r where
asks :: MonadReader r m => (r -> a) -> m a

Documentation

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 :: * -> *) #
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.