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80 lines
2.2 KiB
Haskell
80 lines
2.2 KiB
Haskell
{-# LANGUAGE MultiParamTypeClasses #-}
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-- | Combinators using @CVar@s. These provide many of the helpful
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-- functions found in Control.Concurrent.MVar, but for @CVar@s. Note
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-- that these do not in general mask exceptions, and are not atomic,
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-- being implemented in terms of the primitives in the 'ConcFuture'
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-- and 'ConcCVar' typeclasses.
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module Control.Monad.Conc.CVar
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( -- *Combinators
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newCVar
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, swapCVar
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, isEmptyCVar
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, withCVar
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, modifyCVar
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, modifyCVar_
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-- * Binary semaphores
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-- | A common use of @CVar@s is in making binary semaphores to
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-- control mutual exclusion over a resource, so a couple of helper
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-- functions are provided.
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, lock
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, unlock
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) where
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import Control.Monad (liftM)
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import Control.Monad.Conc.Class
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-- | Create a new @CVar@ containing a value.
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newCVar :: ConcCVar cvar m => a -> m (cvar a)
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newCVar a = do
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cvar <- newEmptyCVar
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putCVar cvar a
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return cvar
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-- | Swap the contents of a @CVar@, and return the value taken.
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swapCVar :: ConcCVar cvar m => cvar a -> a -> m a
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swapCVar cvar a = do
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old <- takeCVar cvar
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putCVar cvar a
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return old
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-- | Check if a @CVar@ is empty.
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isEmptyCVar :: ConcCVar cvar m => cvar a -> m Bool
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isEmptyCVar cvar = do
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val <- tryTakeCVar cvar
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case val of
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Just val' -> putCVar cvar val' >> return True
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Nothing -> return False
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-- | Operate on the contents of a @CVar@, replacing the contents after
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-- finishing.
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withCVar :: ConcCVar cvar m => cvar a -> (a -> m b) -> m b
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withCVar cvar f = do
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val <- takeCVar cvar
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out <- f val
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putCVar cvar val
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return out
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-- | Apply a function to the value inside a @CVar@, and also return a
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-- value.
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modifyCVar :: ConcCVar cvar m => cvar a -> (a -> m (a, b)) -> m b
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modifyCVar cvar f = do
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val <- takeCVar cvar
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(val', out) <- f val
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putCVar cvar val'
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return out
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-- | Modify the contents of a @CVar@.
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modifyCVar_ :: ConcCVar cvar m => cvar a -> (a -> m a) -> m ()
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modifyCVar_ cvar f = modifyCVar cvar $ \a -> (\b -> (b, ())) `liftM` f a
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-- | Put a @()@ into a @CVar@, claiming the lock. This is atomic.
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lock :: ConcCVar cvar m => cvar () -> m ()
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lock = flip putCVar ()
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-- | Empty a @CVar@, releasing the lock. This is atomic.
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unlock :: ConcCVar cvar m => cvar () -> m ()
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unlock = takeCVar
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