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 BSD-style (see the file LICENSE) libraries@haskell.org experimental non-portable (multi-param classes, functional dependencies) Safe Haskell98

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

Methods

ask :: m r Source #

Retrieves the monad environment.

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

Executes a computation in a modified environment.

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

Retrieves a function of the current environment.

Instances