Finally

mr_call(L) = R :-
    [E, _] = solutions(try(mr_call(L))),
    require_complete_switch [E]
    ( E = succeeded(R)
    ; E = failed,
        impure lua_pushboolean(L, no),
        R = 1
    ; E = exception(U),
        impure lua_pushnil(L),
        impure lua_pushuniv(L, U),
        R = 2
    ).


:- pred mr_call(lua::in, int::out) is cc_multi.

mr_call(L,  R) :-
    semipure lua_getupvalue(L, 1),
    semipure univ(F) = lua_touserdata(L, -1) ->
        R = F(L)
    ;
        impure lua_error(L,
        "Called Mercury function without valid func upvalue.").