mtl-2.2.1: Monad classes, using functional dependencies

Copyright(c) Andy Gill 2001, (c) Oregon Graduate Institute of Science and Technology 2001, (c) Jeff Newbern 2003-2007, (c) Andriy Palamarchuk 2007
LicenseBSD-style (see the file LICENSE)
Maintainerlibraries@haskell.org
Stabilityexperimental
Portabilitynon-portable (multi-param classes, functional dependencies)
Safe HaskellSafe-Inferred
LanguageHaskell98

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

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) 
MonadReader r m => MonadReader r (ListT m) 
MonadReader r m => MonadReader r (IdentityT m) 
MonadReader r ((->) r) 
(Monoid w, MonadReader r m) => MonadReader r (WriterT w m) 
(Monoid w, MonadReader r m) => MonadReader r (WriterT w m) 
MonadReader r m => MonadReader r (StateT s m) 
MonadReader r m => MonadReader r (StateT s m) 
MonadReader r m => MonadReader r (ExceptT e m) 
(Error e, MonadReader r m) => MonadReader r (ErrorT e m) 
MonadReader r' m => MonadReader r' (ContT r m) 
Monad m => MonadReader r (ReaderT r m) 
(Monad m, Monoid w) => MonadReader r (RWST r w s m) 
(Monad m, Monoid w) => MonadReader r (RWST r w s m) 

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.