Function Priority

cpp/src/arrow/compute/expression.cc:1094–1099

        if (IsBinaryAssociativeCommutative(*call)) {
          struct {
            int Priority(const Expression& operand) const {
              // order literals first, starting with nulls
              if (operand.IsNullLiteral()) return 0;
              if (operand.literal()) return 1;
              return 2;
            }
            bool operator()(const Expression& l, const Expression& r) const {
              return Priority(l) < Priority(r);
            }