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Control.Monad.Reader.Class |

Portability | non-portable (multi-param classes, functional dependencies)
Stability | experimental
Maintainer | libraries@haskell.org

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://www.cse.ogi.edu/~mpj/) Advanced School of Functional Programming, 1995.

Synopsis

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

ask :: m r
local :: (r -> r) -> m a -> m a

asks :: MonadReader r m => (r -> a) -> m a

Documentation
class Monad m => MonadReader r m | m -> r where

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 r
Retrieves the monad environment.
local :: (r -> r) -> m a -> m a
Executes a computation in a modified environment. Parameters:

- The function to modify the environment.
- `Reader` to run.
- The resulting `Reader`.

Instances

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

asks :: MonadReader r m => (r -> a) -> m a
Retrieves a function of the current environment. Parameters:

- The selector function to apply to the environment.

See an example in Control.Monad.Reader.

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