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Or Else!

• Waiting for alternatives

 atomic (getR r1 3 ‘orElse‘ getR r2 7)

• If the first get successful (no block) : done
• If the second get succeeds: done
• if both block, wait for either one to unblock and retry the whole transaction
• Non-blocking get

 nonBlockGetR :: Resource -> Int -> STM Bool
 nonBlockGetR r i = do { getR r i ; return True }
                   ‘orElse‘ return False

• The caller may create a blocking version

 blockGetR r i =
   do { s <- nonBlockGetR r i;
        if s then return () else retry }

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