Control.Monad.Reader.Class

monads-tf-0.1.0.2: Monad classes, using type families

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 | ross@soi.city.ac.uk
Stability | experimental
Portability | non-portable (type families)
Safe Haskell | Safe
Language | Haskell98

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 m where
  - type EnvType m
  - ask :: m (EnvType m)
  - local :: (EnvType m -> EnvType m) -> m a -> m a
- asks :: MonadReader m => (EnvType m -> a) -> m a

# Documentation

class Monad m => MonadReader m where

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

Associated Types

type EnvType m

Methods

ask :: m (EnvType m)

Retrieves the monad environment.

local

Arguments

:: (EnvType m -> EnvType m) | The function to modify the environment.
-> m a | `Reader` to run in the modified environment.
-> m a |

Executes a computation in a modified environment.

Instances

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

asks

Arguments

:: MonadReader m |
=> (EnvType m -> a) | The selector function to apply to the environment.
-> m a |

Retrieves a function of the current environment.