> On Tuesday, 2003-08-19, 13:18, Simon Marlow wrote:
> > [...]
>
> > Yes, I agree that one shouldn't rely on the "no duplication of work"
> > property. However, folloing this argument we arrive at the
> conclusion that
> > hGetContents is an invalid use of unsafePerformIO. (which
> is something I've
> > been saying for a while now :-).
>
> Can't hGetContents be implemented without unsafePerformIO but with
> unsafeInterleaveIO? Wouldn't this be valid? Am I'm missing
> something here?
unsafeInterleaveIO = return . unsafePerformIO
Well, almost. unsafeInterleaveIO should also give you the guarantee
that the IO in its argument can't be performed *before* the
unsafeInterleaveIO is executed, but I don't think this distinction is
important to the present discussion.
Actually I dislike unsafeInterleaveIO even more than unsafePerformIO,
because it has an implicit laziness assumption. The only purpose of
unsafeInterleaveIO is to "delay" some IO until it is "demanded". But
Haskell lacks a precise definition of "demand", prefering to leave the
evaluation order up to the implementation. Hence, unsafeInterleaveIO is
left in limbo - it might do something useful, but that depends entirely
on your implementation. GHC doesn't guarantee much about
unsafeInterleaveIO at all.
Cheers,
Simon
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