reader package:mtl-prelude

Retrieves a function of the current environment.

The parameterizable reader monad. Computations are functions of a shared environment. The return function ignores the environment, while >>= passes the inherited environment to both subcomputations.

The reader monad transformer, which adds a read-only environment to the given monad. The return function ignores the environment, while >>= passes the inherited environment to both subcomputations.

See examples in Control.Monad.Reader. Note, the partially applied function type (->) r is a simple reader monad. See the instance declaration below.

Transform the value returned by a Reader.

• runReader (mapReader f m) = f .
  runReader m

Transform the computation inside a ReaderT.

• runReaderT (mapReaderT f m) = f .
  runReaderT m

Runs a Reader and extracts the final value from it. (The inverse of reader.)

Execute a computation in a modified environment (a specialization of withReaderT).

• runReader (withReader f m) = runReader m
  . f

Execute a computation in a modified environment (a more general version of local).

• runReaderT (withReaderT f m) =
  runReaderT m . f