[mercury-users] Request for comments on this code

[Peter Schachte]

On Tue, Jul 11, 2006 at 03:46:35PM +1000, Ralph Becket wrote:
> 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.)

Hmmmm, the cc_multi-ness vs. det-ness of the rest of the universe is
an interesting open philosophical question, but I don't think we need
to resolve it.  We'd better not need to because I can't tell the
difference between "the universe could do any number of things, but it
only does one and you can't ask for a retry" and "the universe can
only ever do one thing."  Free will vs. determinism.

> Under that interpretation, IO operations should be cc_multi.

I think it's better to think of it as coroutining between your program
and the rest of the universe.  So when you execute

	io.low_level_action(..., IO0, IO1)

where low_level_action is outside Mercury, what you're really doing is
sending a message to the universe on channel IO0 asking it to perform
the low level action, whereupon it supplies you with a fresh channel
IO1.  This seems like a better model of what's happening, since
between the time I get an IO state and when I use it, the state of the
universe changes.  So it doesn't make sense to think of the IO state
as *holding* the universe, but it's still fine to think of it as a
channel to the universe.

You know, no matter how deep I dig, it still seems to be worms all the
way down.  I don't have a good answer, but I'm still convinced that
it's wrong to include as part of the IO state things that Mercury can
affect without threading an IO state.

-- 
Peter Schachte              The trouble with Communism is the Communists,
schachte at cs.mu.OZ.AU        just as the trouble with Christianity is the
www.cs.mu.oz.au/~schachte/  Christians.
Phone: +61 3 8344 1338          -- H.L. Mencken 
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