Control.Monad.Reader.Class

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.

Documentation

class Monad m => MonadReader r m | m -> r where

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 r

Retrieves the monad environment.

local :: (r -> r) -> m a -> m a

Executes a computation in a modified environment. Parameters:

• The function to modify the environment.

  • Reader to run.
  • The resulting Reader.

Instances

MonadReader s m => MonadReader s (ListT m)

MonadReader r (Reader r)

MonadReader r m => MonadReader r (StateT s m)

MonadReader r m => MonadReader r (StateT s m)

(Monoid w, MonadReader r m) => MonadReader r (WriterT w m)

Monad m => MonadReader r (ReaderT r m)

(Error e, MonadReader r m) => MonadReader r (ErrorT e m)

(Monoid w, MonadReader r m) => MonadReader r (WriterT w m)

MonadReader r' m => MonadReader r' (ContT r m)

Monoid w => MonadReader r (RWS r w s)

Monoid w => MonadReader r (RWS r w s)

(Monoid w, Monad m) => MonadReader r (RWST r w s m)

(Monoid w, Monad m) => MonadReader r (RWST r w s m)

asks :: MonadReader r m => (r -> a) -> m a

Retrieves a function of the current environment. Parameters:

• The selector function to apply to the environment.

See an example in Control.Monad.Reader.