[m-rev.] for review: Parse and check coerce expressions.

[Peter Wang]

On Sat, 13 Mar 2021 23:19:16 +1100 Julien Fischer <jfischer at opturion.com> wrote:
> 
> Hi Peter,
> 
> On Wed, 10 Mar 2021, Peter Wang wrote:
> 
> > This change implements parsing, typechecking, and modechecking of
> > "coerce" expressions from my subtypes proposal, i.e. coerce(Term).
> > Backends currently will abort if asked to generate code for coercions,
> > as subtypes do not yet share data representations with their base types,
> > so most coercions would lead to crashes at runtime anyway.
> 
> ...
> 
> > diff --git a/NEWS b/NEWS
> > index bc556a8a8..f5919d859 100644
> > --- a/NEWS
> > +++ b/NEWS
> > @@ -17,6 +17,11 @@ Changes that may break compatibility
> > 
> > * We have reserved `=<`/2 as a type name.
> > 
> > +* A term with a top level functor `coerce/1` is now treated as a
> 
> s/top level/top-level/

Done.

> > diff --git a/compiler/hlds_goal.m b/compiler/hlds_goal.m
> > index b59e8773e..f98d79628 100644
> > --- a/compiler/hlds_goal.m
> > +++ b/compiler/hlds_goal.m
> > @@ -700,7 +700,11 @@
> >                 % A cast generic_call with two arguments, Input and Output,
> >                 % assigns `Input' to `Output', performing a cast of this kind.
> >                 cast_kind       :: cast_kind
> > -            ).
> > +            )
> > +
> > +    ;       coerce.
> > +            % A coerce expression with two arguments, Input and Output,
> > +            % assigns `Input' to `Output'.
> 
> 
> Did you consider representing coerce expressions by adding a new alternative to
> the cast_kind type and using the cast constructor?  (Most of the code seems to
> treat them identically anyway.)

Yeah, that's how I had it originally. After I switched to this
representation I found a few more places that I had missed, though most
of them would have ended up doing the same thing as cast() anyway.
I'll leave it for now, as it might be useful for backend changes.

> 
> Also, it might be better to use a more specific name, such as 'subtype_coerce',
> for the constructor.

Done.

> The rest looks fine; I'll do a post-commit review of the reference manual changes
> since they was a bit awkward to follow in diff form.

I'll commit it later today so other people can try it out on their own
examples. Thanks for reviewing.

Peter