5 Dec
2007
5 Dec
'07
3:58 a.m.
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. Cheers, Simon