monads-tf-0.1.0.3: 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
- 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.

Minimal complete definition

ask, local

Associated Types

type EnvType m #

Methods

ask :: m (EnvType m) #

Retrieves the monad environment.

local :: (EnvType m -> EnvType m) -> m a -> m a #

Executes a computation in a modified environment.

Instances

MonadReader ((->) r) # |
Associated Types type EnvType ((->) r :: * -> *) :: * # Methods ask :: r -> EnvType ((->) r) # local :: (EnvType ((->) r) -> EnvType ((->) r)) -> (r -> a) -> r -> a #
MonadReader m => MonadReader (MaybeT m) # |
Associated Types type EnvType (MaybeT m :: * -> *) :: * # Methods ask :: MaybeT m (EnvType (MaybeT m)) # local :: (EnvType (MaybeT m) -> EnvType (MaybeT m)) -> MaybeT m a -> MaybeT m a #
MonadReader m => MonadReader (ListT m) # |
Associated Types type EnvType (ListT m :: * -> *) :: * # Methods ask :: ListT m (EnvType (ListT m)) # local :: (EnvType (ListT m) -> EnvType (ListT m)) -> ListT m a -> ListT m a #
(Monoid w, MonadReader m) => MonadReader (WriterT w m) # |
Associated Types type EnvType (WriterT w m :: * -> *) :: * # Methods ask :: WriterT w m (EnvType (WriterT w m)) # local :: (EnvType (WriterT w m) -> EnvType (WriterT w m)) -> WriterT w m a -> WriterT w m a #
(Monoid w, MonadReader m) => MonadReader (WriterT w m) # |
Associated Types type EnvType (WriterT w m :: * -> *) :: * # Methods ask :: WriterT w m (EnvType (WriterT w m)) # local :: (EnvType (WriterT w m) -> EnvType (WriterT w m)) -> WriterT w m a -> WriterT w m a #
MonadReader m => MonadReader (StateT s m) # |
Associated Types type EnvType (StateT s m :: * -> *) :: * # Methods ask :: StateT s m (EnvType (StateT s m)) # local :: (EnvType (StateT s m) -> EnvType (StateT s m)) -> StateT s m a -> StateT s m a #
MonadReader m => MonadReader (StateT s m) # |
Associated Types type EnvType (StateT s m :: * -> *) :: * # Methods ask :: StateT s m (EnvType (StateT s m)) # local :: (EnvType (StateT s m) -> EnvType (StateT s m)) -> StateT s m a -> StateT s m a #
MonadReader m => MonadReader (IdentityT * m) # |
Associated Types type EnvType (IdentityT * m :: * -> *) :: * # Methods ask :: IdentityT * m (EnvType (IdentityT * m)) # local :: (EnvType (IdentityT * m) -> EnvType (IdentityT * m)) -> IdentityT * m a -> IdentityT * m a #
(Error e, MonadReader m) => MonadReader (ErrorT e m) # |
Associated Types type EnvType (ErrorT e m :: * -> *) :: * # Methods ask :: ErrorT e m (EnvType (ErrorT e m)) # local :: (EnvType (ErrorT e m) -> EnvType (ErrorT e m)) -> ErrorT e m a -> ErrorT e m a #
Monad m => MonadReader (ReaderT * r m) # |
Associated Types type EnvType (ReaderT * r m :: * -> *) :: * # Methods ask :: ReaderT * r m (EnvType (ReaderT * r m)) # local :: (EnvType (ReaderT * r m) -> EnvType (ReaderT * r m)) -> ReaderT * r m a -> ReaderT * r m a #
MonadReader m => MonadReader (ContT * r m) # |
Associated Types type EnvType (ContT * r m :: * -> *) :: * # Methods ask :: ContT * r m (EnvType (ContT * r m)) # local :: (EnvType (ContT * r m) -> EnvType (ContT * r m)) -> ContT * r m a -> ContT * r m a #
(Monoid w, Monad m) => MonadReader (RWST r w s m) # |
Associated Types type EnvType (RWST r w s m :: * -> *) :: * # Methods ask :: RWST r w s m (EnvType (RWST r w s m)) # local :: (EnvType (RWST r w s m) -> EnvType (RWST r w s m)) -> RWST r w s m a -> RWST r w s m a #
(Monoid w, Monad m) => MonadReader (RWST r w s m) # |
Associated Types type EnvType (RWST r w s m :: * -> *) :: * # Methods ask :: RWST r w s m (EnvType (RWST r w s m)) # local :: (EnvType (RWST r w s m) -> EnvType (RWST r w s m)) -> RWST r w s m a -> RWST r w s m a #

asks #

Arguments

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

Retrieves a function of the current environment.