At 16:07 11/11/04 +0000, Keith Wansbrough wrote:
Graham Klyne wrote:
> At 12:27 11/11/04 +0000, Ben Rudiak-Gould wrote:
[..]
> >going to be safe, because it's just not the case that
> >
> > x = once (newIORef ())
> > y = x
> >
> >has the same intended meaning as
> >
> > x = once (newIORef ())
> > y = once (newIORef ())
> >
> >No amount of compiler-specific magic is going to fix this.
>
> Ah, yes, I take the point now.
>
> Isn't this generally the case for any value in the IO monad? (Brushing a
> murky area of equivalence; the same IO computation used twice may yield
> different results, so I'm not clear to what extent it is meaningful to say
> that any IO value is the same as any other, including itself, in any
> observable sense.)
No. "getChar" is always "the IO operation that reads a character from
stdin". You can always substitute one instance of "getChar" for
another; you can even say "foo = getChar" and substitute "foo" for
every occurrence of "getChar". A value of type IO a is a
*computation*; its result may change, but the computation itself
cannot.
So you say (and I do agree). But how can I *observe* that they are the
same?
#g
--
------------
Graham Klyne
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