[Haskell-cafe] STM atomic blocks in IO functions
sschuldenzucker at uni-bonn.de
Sat Jan 14 19:05:51 CET 2012
On 01/14/2012 03:55 PM, Ketil Malde wrote:
> "Bryan O'Sullivan"<bos at serpentine.com> writes:
>>> 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
>> 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
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.
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