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Blocking Transactions

• Get: Consumer

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

• retry aborts the transaction, but keeps the log of memory accesses and waits until they change.
• No need for the producer to signal
• The type of retry is (STM a)
• Composability

 atomic (do { getR r1 3; getR r2 7 })

• They may block but will not deadlock
• Any STM action can be robustly composed with any other STM action!

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