[m-dev.] State variable syntax - last call for opinions

[Ralph Becket]

Peter Schachte, Thursday, 31 January 2002:
> 
> That's good, but not what I was talking about.  If I write (using
> Fergus's latest syntax proposal):
> 
> 	foo :-
> 		!:X = 0,
> 		... stuff ...
> 		!:X = !:X + 1.
> 
> that might translate to something like:
> 
> 	foo :-
> 		X1 = 0,
> 		... stuff ...
> 		X5 = X4 + 1.
> 
> The Mercury compiler would issue a singleton variable warning for the
> latter clause because X5 only appears once.  I think it should issue a
> similar warning for the former clause, too.

After more discussion here in the office, we've decided you're right:
- it should be an error for either !.X or !:X to appear without the
  other somewhere in the same clause;
- also it should be an error for !.X to appear before !:X except in the
  head of the clause.

> > > Is it an error to have multiple !:X in the same atom?  I think it
> > > probably should be.  You could give it a sensible meaning, but I think
> > > it would be too confusing to use.
> > 
> > I'd rather stick with a simple, general transformation and let the
> > programmer learn to avoid baroque coding styles the hard way.
> 
> But what does it mean?  For example, how would you translate
> 
> 	!:X = 0,
> 	!:X = !:X + 1,
> 	foo(!.X),

Into

	X3 = 0,
	X4 = X4 + 1,
	foo(X4), X5 = X4,

> or
> 
> 	!:X = 0,
> 	foo(!X, !X).
> 	foo(!.X),

Into

	X7 = 0,
	foo(X7, X8, X7, X8),
	foo(X8),

- Ralph
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