[m-rev.] for review: compare du types separately

[Zoltan Somogyi]

On Mon, 27 Jul 2026 15:41:24 +1000, Peter Wang <novalazy at gmail.com> wrote:
> On Mon, 27 Jul 2026 00:34:29 +0200 "Zoltan Somogyi" <zoltan.somogyi at runbox.com> wrote:
> > --- a/tests/invalid/coerce_type_error.err_exp
> > +++ b/tests/invalid/coerce_type_error.err_exp
> > @@ -26,3 +26,10 @@ coerce_type_error.m:118:   error: cannot coerce `X' from
> >  coerce_type_error.m:118:   `coerce_type_error.wrap_abs(coerce_type_error.citrus)'
> >  coerce_type_error.m:118:   to
> >  coerce_type_error.m:118:   `coerce_type_error.wrap_abs(coerce_type_error.fruit)'.
> > +coerce_type_error.m:125: In clause for predicate `non_subtypes'/2:
> > +coerce_type_error.m:125:   error: cannot coerce `Fruits' from
> > +coerce_type_error.m:125:   `list.list(coerce_type_error.fruit)' to
> > +coerce_type_error.m:125:   `list.list(coerce_type_error.orange_non_fruit)'.
> > +coerce_type_error.m:125:   `coerce_type_error.fruit'/0 is not a subtype.
> > +coerce_type_error.m:125:   `coerce_type_error.orange_non_fruit'/0 is not a
> > +coerce_type_error.m:125:   subtype.
> 
> I think the last two sentences could mislead as to why the coercion is
> invalid. Ideally, it would say something like: Neither `fruit' nor
> `orange_non_fruit' is a subtype of the other.

I see two issues here.

The less important one is that at the moment, each "X is not a subtype"
is a separate coerce_fail that is discovered independently, so replacing two of those
with one message will require post-processing the list of coerce_fails.
Since we already do this (in another form), this is a minor annoyance.

The more significant issue is that "fruit is not a subtype" is a stronger statement
than "fruit is not a subtype of orange_non_fruit", and therefore it gives
more info to the user. I happen to think that extra info may be useful,
at least sometimes. If you don't think so, can you please tell me your reason.

> The other changes look fine.

I followed all your other suggestions. Thanks for the review.

Zoltan.