xmonad-0.9.1: A tiling window manager

MaintainerDon Stewart <dons@galois.com>






(.|.) :: Bits a => a -> a -> a

Bitwise "or"

class Monad m => MonadState s m | m -> s where

get returns the state from the internals of the monad.

put replaces the state inside the monad.


get :: m s

put :: s -> m ()


gets :: MonadState s m => (s -> a) -> m a

Gets specific component of the state, using a projection function supplied.

modify :: MonadState s m => (s -> s) -> m ()

Monadic state transformer.

Maps an old state to a new state inside a state monad. The old state is thrown away.

      Main> :t modify ((+1) :: Int -> Int)
      modify (...) :: (MonadState Int a) => a ()

This says that modify (+1) acts over any Monad that is a member of the MonadState class, with an Int state.

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.


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.

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.

class Monad m => MonadIO m where


liftIO :: IO a -> m a