[Boost-users] Proto transform : fold & fold_tree use case

From: Joel Falcou (joel.falcou_at_[hidden])
Date: 2008-06-17 02:57:02

Hello,

I'm currently trying to write some transforms but I think I horribly 
fail because I must be missing an important point.
Let's say we have a grammar defined as :

struct skeleton :
     or_<
          terminal< bp::_ >
        , bitwise_and<skeleton,skeleton>
        >  {};

What I need to do is build a type out of a given expression. This type 
is in fact a structure containing a tuple of class that represents MPI 
process. So my idea was to write transform that do the following :

* for terminals, extract the argument (which is in fact a functor class) 
and create a process network class instanciation with the argument and a 
PID (basically a mpl::long_<>).
* for the & operator, transform the left and right argument, and build 
the tuple out of both results by merging the contents of both into a 
unique process network.

My biggest concern is to have the PID value to be kept across transform 
call and correctly increased for each terminal. To do so, am I forced to 
use fold (or fold_tree cause in the real code I have more than the sole 
& operator) or is there a simple way to say transform left and right and 
get the resulting types ?

Then, how is State kept through fold_tree or transform iteration ? 
Should I kept the PID current value in an external parameters ?

Thanks in advance.

PS : if more detailed code is needed, I'll try to see what can be 
publicly dispatched here.