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STM Teaser

• Implementation of Producer/Consumer in Haskell with STM
• Integer resource:

 type Resource = TVar Int

• Producer: Put i units of resource into r:

 putR :: Resource -> Int -> STM ()
 putR r i = do { v <- readTVar r
               ; writeTVar r (v+i) }

• Consumer: Acquire i units of resource r, blocking if r holds insufficient units

 getR :: Resource -> Int -> STM ()
 getR r i = do { v <- readTVar r
               ; if (v < i) then retry
                 else writeTVar r (v-i) }

• Client code

 atomic ( getR r x )
 atomic ( putR r x )

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