%---------------------------------------------------------------------------%
% vim: ft=mercury ts=4 sw=4 et
%---------------------------------------------------------------------------%
%
% This is cut-down version of the colipa program by Volker Wysk sent to m-rev
% on 3 January 2024.
%
% Reference:
% https://lists.mercurylang.org/archives/reviews/2024-January/024517.html
%
% Compiling in a debug grade, results in:
%
% Uncaught Mercury exception:
% Software Error: predicate `ll_backend.liveness.require_equal'/4:
%    Unexpected: branches of if-then-else disagree on liveness
% First: ArgDesc_3, Values_4, MDef_5, Default_6,
%    TypeClassInfo_for_argument_21, TypeInfo_22_22
% Rest:  ArgDesc_3, Values_4, MDef_5, Default_6, TypeInfo_22_22
%
% Compilation in non-debug grades succeeds.
%
%---------------------------------------------------------------------------%

:- module bug572.
:- interface.

:- import_module list.
:- import_module string.

%---------------------------------------------------------------------------%

:- type cla ---> cla(string).

:- typeclass argument(T) where [].

:- type argdesc.

%---------------------------------------------------------------------------%

:- pred get_from_default(argdesc::in, list(T)::out)
    is det <= argument(T).

%---------------------------------------------------------------------------%
%---------------------------------------------------------------------------%

:- implementation.

:- import_module map.
:- import_module maybe.
:- import_module exception.
:- import_module type_desc.

%---------------------------------------------------------------------------%
%---------------------------------------------------------------------------%

:- type argdesc
    --->    some [T] argdesc(
                argdesc_default :: maybe(T)
            ).

%---------------------------------------------------------------------------%

get_from_default(ArgDesc, Values) :-
    MDef = ArgDesc ^ argdesc_default,
    (
        MDef = yes(Default),
        ( if
            private_builtin.typed_unify(Default, Val0)
        then
            Values = [Val0]
        else
            ValuesTypeDesc = type_of(Values),
            type_ctor_and_args(ValuesTypeDesc, _, TArgs),
            (
                TArgs = [_],
                throw("mismatch")
            ;
                TArgs = [],
                throw("no type args")
            ;
                TArgs = [_, _ | _],
                throw("too many type args")
            )
        )
    ;
        MDef = no,
        Values = []
    ).

%---------------------------------------------------------------------------%