[m-rev.] for review: subtypes

[Mark Brown]

Hi Peter,

On Mon, Jan 6, 2014 at 2:19 PM, Peter Wang <novalazy at gmail.com> wrote:
> On Tue, 31 Dec 2013 05:05:15 +1100, Mark Brown <mark at mercurylang.org> wrote:
>> Hi everyone,
>>
>> I have had a shot at implementing something along the lines of the old
>> subtypes proposal. It is for review by anyone. The log message is
>> attached, and the diff can be found at
>>
>> https://github.com/markbrown/mercury/
>>
>> in the subtypes branch. Please note I am not currently subscribed to this list.
> ...
>>
>> Another design issue is that subtypes are currenly only allowed to be used
>> in pred and func declarations (including typeclass methods).  Alternative
>> designs could include:
>>
>> (1) Allowing subtypes to be used in ordinary type declarations, inferring a
>> ground inst for every type that refers, directly or indirectly, to a subtype.
>
> This seems a major restriction.

Indeed, I hit it already in parsing_utils.

Do you think the alternative is better? I mentioned some concerns
about it already, so I wanted feedback before looking further.

>  I think the main use I would have for
> subtypes is to recover the inst of a value after passing through some
> container (which doesn't use polymorphic modes).

That's what polymorphic modes are for. Why don't you want to use them?

(I guess the real question is why the standard library doesn't provide them.)

>
> construct would be even more complicated, no doubt.

The types in type_desc currently provide access to type information
only, not inst information. I wasn't planning to change that at all,
even in the alternative design, so construct, etc, won't be affected.

Cheers,
Mark.