[mercury-users] Request for comments on this code

[Ralph Becket]

Ian MacLarty, Tuesday, 11 July 2006:
> On Tue, Jul 11, 2006 at 03:46:35PM +1000, Ralph Becket wrote:
> > Here's my two worms' worth:
> > 
> > I've always assumed that the IO state corresponded to a representation
> > of part of the world, including parts of the state of the computer
> > running the computation.
> > 
> > When an IO operation is invoked, the universe gets to manipulate the IO
> > state just before and just after the IO operation is carried out.
> > 
> > That is, when I write
> > 
> > 	io.print(..., !IO),
> > 
> > what really happens is
> > 
> > 	universe_gets_a_turn(!IO),
> > 	io.print(..., !IO),
> > 	universe_gets_a_turn(!IO),
> > 
> > What the universe does, however, is cc_multi (i.e., for any IO state
> > there are many possible successor IO states, but we only get one of
> > them and we can't backtrack.)
> > 
> 
> Does the one that gets chosen happen to be the IO state that's fed as
> input to the next IO primitive?

Yes.

> If so then I think your understanding
> is quite similar to mine, except you view it as cc_multi and I view it as
> deterministic.

I don't view it as morally deterministic because, under this reading,
IO operations cannot implement functional relationships between IO
states.

> > Is there a problem with this view (i.e., IO is really cc_multi, but we
> > actually lose something if we don't pretend it's det)?
> 
> I don't think so, since a particular IO state exists only once, so you
> can always only get one resulting IO state from calling an IO primitive,
> effectively making the operation det.

I don't follow you here.
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