[m-dev.] alternative syntax for pred declarations.

[Fergus Henderson]

On Mon, Dec 01, 1997 at 04:16:25PM +1100, Thomas Charles CONWAY wrote:
> Fergus Henderson, you write:
> > 
> > This is getting better, but there are still some problems.
> > 
> > The syntax
> > 
> > 	:- mode this_parser :: parser.
> > 
> > already has a meaning.
> > It is the same as the (undocumented) alternative
> > 
> > 	:- mode this_parser == parser.
> 
> Gosh! How about changing its meaning then?

That would have some problems, starting with backwards compatibility.

> Is it necessary to have two different ways of writting the same
> thing (especially since it is something that is written infrequently,
> and neither syntax is can really be considered easier to use than
> the other)?

The `::' syntax was chosen because it works better than `==',
because `==' doesn't really have the right precedence.

In particular,

	:- mode out == ground -> free.

is a syntax error, because it parses as

	:- mode (out == ground) -> free.

whereas

	:- mode out :: ground -> free.

parses correctly.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>   WWW: <http://www.cs.mu.oz.au/~fjh>  
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