mtl-tf-0.1: Monad transformer library using type families

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 m 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 (EnvType m)Source

Retrieves the monad environment.

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

Executes a computation in a modified environment. Parameters:

  • The function to modify the environment.
  • Reader to run.
  • The resulting Reader.


MonadReader ((->) r) 
MonadReader (Reader r) 
MonadReader m => MonadReader (ListT m) 
Monad m => MonadReader (ReaderT r m) 
(Error e, MonadReader m) => MonadReader (ErrorT e m) 
MonadReader m => MonadReader (StateT s m) 
MonadReader m => MonadReader (StateT s m) 
(Monoid w, MonadReader m) => MonadReader (WriterT w m) 
(Monoid w, MonadReader m) => MonadReader (WriterT w m) 
MonadReader m => MonadReader (ContT r m) 
Monoid w => MonadReader (RWS r w s) 
Monoid w => MonadReader (RWS r w s) 
(Monoid w, Monad m) => MonadReader (RWST r w s m) 
(Monoid w, Monad m) => MonadReader (RWST r w s m) 

asks :: MonadReader m => (EnvType m -> 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.

type family EnvType m Source