On Fri, Feb 22, 2008 at 09:41:46AM +0000, Simon Marlow wrote:
I propose that unsafeDupablePerformIO be renamed to unsafePerformIO, since it satisfies all of the properties guaranteed of unsafePerformIO. GHC's unsafePerformIO guarantees more, and should be called unsafePerformIOAtMostOnce or something.
That's certainly a defensible position, but I'll present a counter-argument.
If you've managed to use unsafePerformIO in a way that works on a single processor, then you will probably be tempted to assume that it will work on a multiprocessor too. Currently unsafePerformIO tries to make that true (although it's not foolproof, and it's quite expensive).
Look at all that stuff in the docs for unsafePeformIO about -fno-cse and let-floating (I think it goes too far, in fact - if your use of unsafePerformIO is that fragile then you're doing something wrong). If unsafePerformIO was the dupable version by default, then all that goes out of the window if you happen to be running with two threads on an SMP. And if you're writing a library, you have to assume the worst and go for the AtMOstOnce version - who would remember to do that?
Better for the default version to be safe in this respect, IMO. The bugs we'd get from this would be really hard to track down.
Also, unsafePerformIO is in the FFI specification, even though it isn't fully specified with respect to multi-processing I think there was some expectations of what 'unsafePerformIO' meant, for better or worse. John -- John Meacham - ⑆repetae.net⑆john⑈