Re: [Boost-users] [proto] : Proto transform with state

On 11/17/2010 1:47 AM, Manjunath Kudlur wrote:

I am trying to create a transform where I replace occurrences of proto::terminal<term> with proto::terminal >. I want N to increment for every proto::terminal<term> encountered. So far I have the following code :

<snip>

I pass in mpl::int_<0>() as the initial state and call mpl::next when I match proto::terminal<term>. As the output would indicate, this is not working. Every time I think I understand proto transforms, something basic like this stumps me. I will be grateful for any hints on how to accomplish this.

Don't feel bad. This is EXTREMELY hard to do in Proto. :-( Most algorithms only return one piece of information at each step. This task requires returning two: the transformed expression and the new state. Worse, after transforming a left child, the new state needs to be used as input while transforming the right. Like I said, EXTREMELY hard. Since your algorithm needs to return two pieces of information, it needs to return a std::pair containing the new expression and the updated state variable. And when transforming a non-terminal, you need to use the fold algorithm to propagate the state from one invocation to the next (again, bundling and unbundling into a pair as necessary). Fold can build a fusion::vector of transformed children, which then needs to be unpacked into an expression. (This step wouldn't be necessary if Proto expressions were extensible Fusion sequences. <sigh>) See the attached code. I wish I had a better answer. It sure would be nice to generalize this for other times when new state needs to bubble up and back down. -- Eric Niebler BoostPro Computing http://www.boostpro.com