monads-fd-0.0.0.0: Monad classes, using functional dependenciesSource codeContentsIndex
Control.Monad.Reader.Class
Portabilitynon-portable (multi-param classes, functional dependencies)
Stabilityexperimental
Maintainerlibraries@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 Control.Monad.State.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
asks :: MonadReader r m => (r -> a) -> m a
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.
localSource
::
=> r -> rReader to run in the modified environment.
-> m a
-> m a
Executes a computation in a modified environment.
show/hide Instances
MonadReader r m => MonadReader r (ListT m)
MonadReader r ((->) r)
(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 (StateT s m)
MonadReader r m => MonadReader r (StateT s m)
(Error e, MonadReader r m) => MonadReader r (ErrorT e m)
MonadReader r' m => MonadReader r' (ContT r m)
Monad m => MonadReader r (ReaderT r m)
(Monad m, Monoid w) => MonadReader r (RWST r w s m)
(Monad m, Monoid w) => MonadReader r (RWST r w s m)
asksSource
:: MonadReader r m
=> r -> a
-> m a
Retrieves a function of the current environment.
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