dsl_arithmetic_5_eval.h

using env = std::map<id, nb>;

auto eval_alg(env const& env)
{
   return [&env] (expression_r<int> const& e)
   {
      if (auto* o = get_as_add(e))
         return boost::accumulate(o->rands(), 0, std::plus<int>());
         
      if (auto* o = get_as_mul(e))
         return boost::accumulate(o->rands(), 1, std::multiplies<int>());
      
      if (auto* v = get_as_var(e)) return env.find(*v)->second;
      if (auto* i = get_as_cst(e)) return *i;
      throw_missing_pattern_matching_clause();
   };
}

int eval(env const& env, expression const& expr)
{
   return cata<int>(eval_alg(env), expr);
}