Greenspanning like a boss

%-----------------------------------------------------------------------------%
%
% Lua Variables.
%

    % This type represents the Mercury equivalent to Lua's variable list
    % espression, used to pass varadic function arguments and return
    % values.

:- type vars
    --->    some [T] (var(T))
    ;   some [T] (cons_var(T, vars))
    where equality is unify_vars.

    % Due to nil's special status as the abscence of value, a single
    % nil value is passed in place of the empty list and will unify
    % with lists of nil.
    %
:- pred unify_vars(vars::in, vars::in) is det.

:- implementation. % TODO: Move to main implementation

unify_vars(var(T1), var(T2)) :- dynamic_cast(T1, T2).

unify_vars(cons_var(T1, Vars1), cons_var(T2, Vars2) :-
    dynamic_cast(T1, T2),
    unify_vars(Vars1, Vars2).

unify_vars(var(T1), cons_var(T2, var(nil))) :- dynamic_cast(T1, T2).

unify_vars(cons_var(T1, var(nil)), var(T2)) :- dynamic_cast(T1, T2).

:- interface.

:- func vars(T, vars) = vars.
:- mode vars(in, in) = out is det.
:- mode vars(unused, out) = in is det.
:- mode vars(out, out) = in is semidet.

:- implementation. % TODO: Move to main implementation

vars(T::in, Vars::in) = ('new cons_var'(T, Vars)::out).

vars(_, Vars) = cons_var(_, Vars).

vars(T1, Vars) = cons_var(T2, Vars) :- dynamic_cast(T1, T2).