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Resource Manager

• Resource type

 type Resource = TVar Int

• Put: Producer

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

• Transactional usage

atomic takes a memory transaction, of type STM a, and delivers an I/O action that, when performed, runs the transaction atomically with respect to all other memory transactions.

 main = do { ...; atomic (putR r 3); ... }

• The type system prevents IO inside transactions
• It prevents calling STM actions outside atomic block

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