[m-users.] Inst problem

[Mark Brown]

On Sat, Nov 1, 2025 at 9:45 PM Volker Wysk <post at volker-wysk.de> wrote:
>
> Hi!
>
> I have this:
>
> ------------
> :- type iosupp.res(T)
>     ---> ok(T)
>     ;    error(iosupp.error).
>
> :- inst iosupp.res(I)
>     ---> ok(I)
>     ;    error(ground).
>
> :- type path ---> path(
>     abs :: bool,
>     comps :: list(string)
> ).
>
> :- inst abs_path == bound(path(bound(yes), ground)).
>
> :- pred absolute_path_p(path, iosupp.res(path),   io, io).
> :- mode absolute_path_p(in,   out(res(abs_path)), di, uo) is det.
>
> :- pred current_directory_p(
>     iosupp.res(path)::out(iosupp.res(abs_path)),
>     io::di, io::uo
> ) is det.
>
> absolute_path_p(Path, Res, !IO) :-
>     (
>         if   Path ^ abs = yes
>         then Res = iosupp.ok(Path)
>         else dirsupp.current_directory_p(Res1, !IO),
>              (
>                  Res1 = ok(Cwd),
>                  Res  = ok(path(yes, Cwd ^ comps ++ Path ^ comps))
>              ;
>                  Res1 = iosupp.error(Err),
>                  Res  = iosupp.error(Err)
>              )
>     ).
> ------------
>
> And I get this error message from the compiler:
>
> iosupp.m:1065: In clause for `absolute_path_p(in,
> out(iosupp.res(iosupp.abs_path)), di, uo)':
> iosupp.m:1065:   mode error: argument 2 had the wrong instantiatedness.
> iosupp.m:1065:   Final instantiatedness of `Res' was
> iosupp.m:1065:     unique(
> iosupp.m:1065:       error(ground)
> iosupp.m:1065:     ;
> iosupp.m:1065:       ok(
> iosupp.m:1065:         bound(
> iosupp.m:1065:           path(ground, ground)
> iosupp.m:1065:         )
> iosupp.m:1065:       )
> iosupp.m:1065:     ),
> iosupp.m:1065:   expected final instantiatedness was
> iosupp.m:1065:     named inst res(abs_path)
> iosupp.m:1065:     which expands to
> iosupp.m:1065:       bound(
> iosupp.m:1065:         error(ground)
> iosupp.m:1065:       ;
> iosupp.m:1065:         ok(
> iosupp.m:1065:           named inst abs_path
> iosupp.m:1065:           which expands to
> iosupp.m:1065:             bound(
> iosupp.m:1065:               path(
> iosupp.m:1065:                 bound(
> iosupp.m:1065:                   yes
> iosupp.m:1065:                 ),
> iosupp.m:1065:                 ground
> iosupp.m:1065:               )
> iosupp.m:1065:             )
> iosupp.m:1065:         )
> iosupp.m:1065:       ).
>
>
> I think I understand the final instantiatedness of `Res', and that it
> doesn't match the mode line for absolute_path_p. But how *should* it be
> done?

I would do this:

:- type path
    --->    absolute_path(list(string))
    ;       relative_path(list(string)).

Or something else that better reflects what the type actually is :-)

Cheers,
Mark

>
> TIA,
> Volker
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