[Haskell-cafe] One-shot? (was: Global variables and stuff)

[Graham Klyne]

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
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Graham Klyne
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