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{- state monad support
-
- Copyright 2012 Joey Hess <joey@kitenet.net>
-
- Licensed under the GNU GPL version 3 or higher.
-}
{-# LANGUAGE PackageImports #-}
module Utility.State where
import "mtl" Control.Monad.State.Strict
{- Modifies Control.Monad.State's state, forcing a strict update.
- This avoids building thunks in the state and leaking.
- Why it's not the default, I don't know.
-
- Example: changeState $ \s -> s { foo = bar }
-}
changeState :: MonadState s m => (s -> s) -> m ()
changeState f = do
x <- get
put $! f x
{- Gets a value from the internal state, selected by the passed value
- constructor. -}
getState :: MonadState s m => (s -> a) -> m a
getState = gets
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