Re: [Haskell-cafe] STM atomic blocks in IO functions

On 01/14/2012 03:55 PM, Ketil Malde wrote:

"Bryan O'Sullivan" writes:

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The question is a simple one. Must all operations on a TVar happen within *the same* atomically block, or am I am I guaranteed thread safety if, say, I have a number of atomically blocks in an IO function.

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If you want successive operations to see a consistent state, they must occur in the same atomically block.

I'm not sure I understand the question, nor the answer? I thought the idea was that state should be consistent on the entry and exit of each "atomically" block. So you can break your program into multiple transactions, but each transaction should be a semantically complete unit.

I think "consistent state" here means that you can be sure no other thread has modified a, say, TVar, within the current 'atomically' block. E.g. for MVars, you could /not/ be sure that void (takeMVar mvar) >> putMVar mvar 5 won't block if mvar is full at the beginning, because a different thread might put to mvar between the two actions. However, in atomically $ void (takeTVar tvar) >> putTVar tvar 5 , this is not possible, the action after 'atomically' won't be influenced by any other threads while it's running, hence the name. -- Steffen