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
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