Hello, I want to create a DSL where users are allowed to create "free" variables and then use them are parameters and local variables in their mini-program. For example, I want to have something like int_ a,b,c; Program(&a,b) [c = b*2, a = c] The parameter with the '&' prefix represents output, the ones without represent input. So, Program(&a, b)[...] should behave as a function object with prototype void foo(int&, const int), and I should be able to do the following : int ra; BOOST_AUTO(pr, Program(&a,b) [c = b*2, a = c]); pr(ra, 4); I am seeking some advice on implementing the "evaluator" for this language. In particular, where do I store the values corresponding to these free variables? To draw parallels with Boost.Phoenix, in phoenix, arg1, arg2.. etc are uniquely typed, and the evaluator just creates a fusion::vector<...> with the values passed in, and argN evaluation just returns fusion::at_c<N>(env.args()). In my case, I could create a unique ID for each free variable, and create a std::map from ID to value, and use that as the environment. Evaluator of the free variables can then look up that map to find values. But I consider this too heavy weight. In contrast, the fusion::vector<...> would incur very little overhead during run time. I will be grateful for any suggestions on other ways to implement the evaluator. Thanks in advance, Manjunath