Portability  nonportable (multiparam classes, functional dependencies) 

Stability  experimental 
Maintainer  libraries@haskell.org 
 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 subcomputations 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 HigherOrder Polymorphism/, Mark P Jones (http://www.cse.ogi.edu/~mpj/) Advanced School of Functional Programming, 1995.
 class Monad m => MonadReader r m  m > r where
 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.
Retrieves the monad environment.
local :: (r > r) > m a > m aSource
Executes a computation in a modified environment. Parameters:
 The function to modify the environment.

Reader
to run.  The resulting
Reader
.
(Monad m, MonadTrans t, Monad (t (ReaderT r m)), Monad (t (StateT r m))) => MonadReader r (t (ReaderT r m))  
(MonadTrans t, Monad (t (Reader r)), Monad (t (State r))) => MonadReader r (t (Reader r))  
MonadReader r (Reader r)  
MonadReader r ((>) r)  
Monad m => MonadReader r (ReaderT r m) 
asks :: MonadReader r m => (r > a) > m aSource
Retrieves a function of the current environment. Parameters:
 The selector function to apply to the environment.
See an example in Control.Monad.Reader.