(*
 * Cf. ./sum-list.rsl.  Getting more twinges that multiple inheritance a la
 * Java interfaces subsumes OBJ-style theories and views.
 *)

object SummableList = SummableObject*;
object SummableObject = i:integer or r:real;

(*
 * The need for this version of Sum, as opposed to just being able to use
 * built-in '+', is I believe obviatelable if we define '+' properly by putting
 * it's sig in the Level 0 symtab rather than typechecking its args as a
 * special case.
 *)
op Sum(x:SummableObject, y:SummableObject) -> real =
    if (x?.i) and (y?.i) then x.i + y.i
    else if (x?.i) and (y?.r) then x.i + y.r
    else if (x?.r) and (y?.i) then x.r + y.i
    else x.r + y.r;

function SumList(sl:SummableList)->real =
    if sl = nil
    then 0
    else sl[1] + SumList(sl[2:#sl]);	-- ERROR in V3, per earlier comment
					-- above definition of function Sum
	-- !! FIXED 3dec09.  See CVS log and admin/LOG entries of this date.

function SumList2(sl:SummableList)->real =
    if sl = nil
    then 0
    else Sum(sl[1], SumList(sl[2:#sl]));

op main() = (
    Sum(1,2);
    Sum("1","2");			-- ERROR, presumably always
);