[m-rev.] for review: Parse and check coerce expressions.
novalazy at gmail.com
Mon Mar 15 11:23:08 AEDT 2021
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/
> > 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.
> 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.
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