Control.Monad.Reader.Class

mtl-2.2.2: 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
License | BSD-style (see the file LICENSE)
Maintainer | libraries@haskell.org
Stability | experimental
Portability | non-portable (multi-param classes, functional dependencies)
Safe Haskell | Safe
Language | Haskell2010

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 r m | m -> r where
  - ask :: m r
  - local :: (r -> r) -> m a -> m a
  - reader :: (r -> 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.

Minimal complete definition

(ask | reader), local

Methods

ask :: m r #

Retrieves the monad environment.

local #

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 #

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) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: MaybeT m r # local :: (r -> r) -> MaybeT m a -> MaybeT m a # reader :: (r -> a) -> MaybeT m a #
MonadReader r m => MonadReader r (ListT m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: ListT m r # local :: (r -> r) -> ListT m a -> ListT m a # reader :: (r -> a) -> ListT m a #
(Monoid w, MonadReader r m) => MonadReader r (WriterT w m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: WriterT w m r # local :: (r -> r) -> WriterT w m a -> WriterT w m a # reader :: (r -> a) -> WriterT w m a #
(Monoid w, MonadReader r m) => MonadReader r (WriterT w m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: WriterT w m r # local :: (r -> r) -> WriterT w m a -> WriterT w m a # reader :: (r -> a) -> WriterT w m a #
MonadReader r m => MonadReader r (StateT s m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: StateT s m r # local :: (r -> r) -> StateT s m a -> StateT s m a # reader :: (r -> a) -> StateT s m a #
MonadReader r m => MonadReader r (StateT s m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: StateT s m r # local :: (r -> r) -> StateT s m a -> StateT s m a # reader :: (r -> a) -> StateT s m a #
MonadReader r m => MonadReader r (IdentityT m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: IdentityT m r # local :: (r -> r) -> IdentityT m a -> IdentityT m a # reader :: (r -> a) -> IdentityT m a #
MonadReader r m => MonadReader r (ExceptT e m) # | Since: mtl-2.2
Instance details Defined in Control.Monad.Reader.Class Methods ask :: ExceptT e m r # local :: (r -> r) -> ExceptT e m a -> ExceptT e m a # reader :: (r -> a) -> ExceptT e m a #
(Error e, MonadReader r m) => MonadReader r (ErrorT e m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: ErrorT e m r # local :: (r -> r) -> ErrorT e m a -> ErrorT e m a # reader :: (r -> a) -> ErrorT e m a #
MonadReader r' m => MonadReader r' (ContT r m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: ContT r m r' # local :: (r' -> r') -> ContT r m a -> ContT r m a # reader :: (r' -> a) -> ContT r m a #
Monad m => MonadReader r (ReaderT r m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: ReaderT r m r # local :: (r -> r) -> ReaderT r m a -> ReaderT r m a # reader :: (r -> a) -> ReaderT r m a #
MonadReader r ((->) r :: Type -> Type) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: r -> r # local :: (r -> r) -> (r -> a) -> r -> a # reader :: (r -> a) -> r -> a #
(Monad m, Monoid w) => MonadReader r (RWST r w s m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: RWST r w s m r # local :: (r -> r) -> RWST r w s m a -> RWST r w s m a # reader :: (r -> a) -> RWST r w s m a #
(Monad m, Monoid w) => MonadReader r (RWST r w s m) # |
Instance details Defined in Control.Monad.Reader.Class Methods ask :: RWST r w s m r # local :: (r -> r) -> RWST r w s m a -> RWST r w s m a # reader :: (r -> a) -> RWST r w s m a #

asks #

Arguments

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

Retrieves a function of the current environment.