Hi,
On 2009/03/16, at 10:04, wren ng thornton wrote:
> next r = do n <- readIORef r
> writeIORef r (n+1)
> return n
Now, if I use unsafeInterleaveIO:
> main = do r <- newIORef 0
> x <- do a <- unsafeInterleaveIO (next r)
> b <- unsafeInterleaveIO (next r)
> return (a,b)
> ...
The values of a and b in x are entirely arbitrary, and are only set
at the point when they are first accessed. They're not just
arbitrary between which is 0 and which is 1, they could be *any*
pair of values (other than equal) since the reference r is still in
scope and other code in the ... could affect it before we access a
and b, or between the two accesses.
OK, the values of a and b depend how to deconstruct x.
Moreover, let's have two pure implementations, f and g, of the same
mathematical function. Even if f and g are close enough to
correctly give the same output for inputs with _|_ in them, we may
be able to observe the fact that they arrive at those answers
differently by passing in our x. Given that such observations are
possible, it is no longer safe to exchange f and g for one another,
despite the fact that they are pure and give the same output for
all (meaningful) inputs.
Hm, Does it means that in optimization, a compiler may replace
implementation of a pure function that have different order of
evaluation, so order of actions would be different in some environments?
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