[m-rev.] for review: state vars syntax sugar

[Mark Brown]

On 25-Jan-2007, Ben Schmidt <b.schmidt at ugrad.unimelb.edu.au> wrote:
> >(3) For consistency in the concrete syntax, I think a state-variable
> >mode pair should also be preceeded by !.  That is, the following should
> >be equivalent:
> >
> >:- pred foo(!bar::!(in, out)) is det.
> >
> >:- pred foo(!bar).
> >:- mode foo(!(in, out)) is det.
> >
> >:- pred foo(bar, bar).
> >:- mode foo(in, out) is det.
> >
> >The ! symbol immediately alerts the reader to the fact that state
> >variables are being used.  Just using a (_, _) pair for the modes
> >omits this helpful visual clue.
> 
> There was lengthy discussion about this (thread: [m-dev.] syntactic 
> suggar suggestion for state variables). I think the consensus of the 
> list was *not* to use ! in the mode part of the declaration, but to have 
> the syntax as Ondrej implemented it. Reasoning:
> 
> - Using !(in,out) looks like it should expand to (in0,out0),(in,out), 
> i.e. four arguments, or two pairs, which it doesn't.
> - If using ! as a functor for modes, it should take an inst argument if 
> anything, but what is wanting to be done isn't parameterisable in terms 
> of an inst argument (at least at present).
> - The syntax as implemented by Ondrej is the one suggested in the 
> existing compiler code.
> 
> Others can confirm that this was the consensus, or reopen the 
> argument.... My vote is for the way Ondrej has done it.

The changes to the reference manual really should be reviewed first, and
hopefully we can reach consensus on that.  *Then* we can more easily review
the code.

Cheers,
Mark.

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