reader package:relude

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 m >>= k 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 m >>= k 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.

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

This function helps with optimizing performance when working with the ReaderT transformer. If you have code like below, that is called in a loop

step :: Instruction -> ReaderT Config IO Result
step instruction = case instruction of
Add -> do stuff ...
Del -> do stuff ...

you can improve performance of your Haskell applications by using etaReaderT in the following way:

step :: Instruction -> ReaderT Config IO Result
step instruction = etaReaderT $ case instruction of
Add -> do stuff ...
Del -> do stuff ...

For a detailed explanation, refer to the following blog post:

• Faster Winter 5: Eta-Expanding ReaderT (by Joachim Breitner)

Shorter and more readable alias for flip runReader.

>>> usingReader 42 $ asks (+5)
47

Shorter and more readable alias for flip runReaderT.

>>> usingReaderT 42 $ asks (+5)
47