mmtl-0.1: Modular Monad transformer library

Portabilitynon-portable (multi-param classes, functional dependencies)



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:
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 ( Advanced School of Functional Programming, 1995.



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.


ask :: m rSource

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.