[m-dev.] Reopened: state vars syntax sugar

[Peter Schachte]

Gregory James DUCK wrote:
> :- pred main(!io::di) is det.	or
> :- pred main(!io::!di) is det	(?)
> I don't think it looks too bad.  Actually, now that I think about it I'd prefer
> (!io::!di) over (!io::di).

I think the real alternative would be !(io::di) rather than (!io::di).  But I
agree, both look pretty good.

Each of them has lexical problems.  !Type::!Mode requires us to add a new
operator ::! and to separately handle !Type :: !Mode (I really don't think you
want to support only one of them).  With !(io::di) you'll have people who want
to write !io::di.  Either of these problems can be worked around, and there's
really not a lot between them.

But the bigger question is what !Mode means.  Can you use it with any old mode?
 What would !out mean?  How about !uo?  or !(bound(a;b;c)>>bound(a;b))?  It'd
be nicer to have a syntax which does not permit semantic errors.  (I know, the
current mode syntax permits semantic errors like ground>>free, but if we can
come up with a syntax that makes semantic errors, that would be preferable.
The !Inst syntax would work with any inst.

> I like this syntax mainly because it is concise (which is surely one of the
> main reasons for introducing syntactic sugar).  The only funny thing is
> understanding that !di or !in also an "out" mode as well

It's a lot better than !Type::(Mode1, Mode2), but it's not any more concise
than !Inst.  I think it's a bit less natural to think about inferring the mode
of the other argument from the one that's specified than it is to think of the
argument as a single state variable argument having a value whose inst is
specified as !Inst in the predicate's mode.

-- 
Peter Schachte              I worry that 10 or 15 years from now, [my child]
schachte at cs.mu.OZ.AU        will come to me and say 'Daddy, where were you
www.cs.mu.oz.au/~schachte/  when they took freedom of the press away from
Phone: +61 3 8344 1338      the Internet?' -- Mike Godwin
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