Control.Monad.Interface.Reader

This module exports:

1. The MonadReader type class and its operations reader, ask and local.
2. An instance of MonadReader for the -> type.
3. Instances of MonadReader for the relevant monad transformers from the transformers package (ReaderT', lazy RWST and strict RWST).
4. A universal pass-through instance of MonadReader for any existing MonadReader wrapped by a MonadLayer.
5. The utility operations asks.

Documentation

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

The 'MonadReader interface monad represents computations which can read values from a shared environment, pass values from function to function and execute sub-computations in a modified environment. Using the MonadReader interface for such computations is often clearer and easier than using the MonadState interface.

Minimal complete definition: local and one of either reader or ask.

Methods

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

Embed a simple reader action into the monad.

ask :: m r

Retrieves the monad environment.

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

Executes a computation in a modified environment.

Instances

(MonadLayer m, MonadReader r (Inner m)) => MonadReader r m

MonadReader r ((->) r)

(MonadReader r f, MonadReader r g) => MonadReader r (Product f g)

Monad m => MonadReader r (ReaderT r m)

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

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

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

Retrieves a function of the current environment.