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Showing content from http://hackage.haskell.org/packages/archive/mtl/2.0.1.0/doc/html/Control-Monad-Reader-Class.html below:

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.

Documentation

class Monad m => MonadReader r m | m -> r whereSource

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

Methods

ask :: m rSource

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.

asks Source