On Dec 5, 2007, at 3:58 AM, Simon Marlow wrote:
Jan-Willem Maessen wrote:
Consider this:
do x <- newIVar let y = readIVar x writeIVar x 3 print y
(I wrote the let to better illustrate the problem, of course you can inline y if you want). Now suppose the compiler decided to evaluate y before the writeIVar. What's to prevent it doing that? Nothing in the Haskell spec, only implementation convention. Nope, semantics. If we have a cyclic dependency, we have to respect it---it's just like thunk evaluation order in that respect.
Ah, so I was thinking of the following readIVar:
readIVar = unsafePerformIO . readIVarIO
But clearly there's a better one. Fair enough.
Hmm, so unsafePerformIO doesn't deal with any operation that blocks? I'm wondering about related sorts of examples now, as well: do x <- newIVar y <- unsafeInterleaveIO (readIVarIO x) writeIVar x 3 print y Or the equivalent things to the above with MVars. -Jan
Cheers, Simon