[mercury-users] inst of list in non-empty branch

[doug.auclair at logicaltypes.com]

Dear Ralph (and all), you and I wrote:

>> print_kids(Element, Kids@[_|_], !IO) :-
>>          format("The child element%s of ", [s(singular_plural(Kids))], !IO),
>>          print(Element, !IO),
>>          is_are(Kids, !IO),
>>          print(Kids, !IO),
>>          nl(!IO).

>The mode for format/4 is pred(in, in, di, uo), so while the compiler
>knows that Kids has an inst matching non_empty_list before the call to
>format, it only knows it has inst ground afterwards.

Okay.  I think, however, that the mode for Kids is being set to just 'in'
anyway, because when I reduce the clause to:

print_kids(_, Kids@[_|_], !_IO) :-
        TheS = singular_plural(Kids).

-- recall that singular_plural/1's mode is in(non_empty_list) -- I still
get the same error:

test_xml_kids.m:071: In clause for `print_kids(in, in, di, uo)':
test_xml_kids.m:071:   in argument 1 of call to function
test_xml_kids.m:071:   `test_xml_kids.singular_plural'/1:
test_xml_kids.m:071:   mode error: variable `Kids' has instantiatedness
test_xml_kids.m:071:   `ground',
test_xml_kids.m:071:   expected instantiatedness was `bound(list.'[|]'(ground,
test_xml_kids.m:071:   ground))'.

This leads me to suspect that the mode for Kids is set to only 'in', even
in this non-empty-list clause.  ... I have seen the mode system set the
inst properly for other user-defined discriminated union types, particularly
when implementing different modes for different clauses (as illustrated in
the refman, sect 4.4) so it's puzzling why this particular example is being 
uncooperative.

Is there a pragma or assertion where I can assign the inst to Kids? Is
there some call-out-to-C hackery that would force the inst assignment?

... or, if I had some time, how would you recommend I dig into the mode
system to implement changes for "mode-refinement-only" to the compiler?

Sincerely,
Doug Auclair

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