Proto for Boost mailing list page: [proto] Transform result_of nightmare and preserving terminal identity

Subject: [proto] Transform result_of nightmare and preserving terminal identity
From: Agustín K-ballo Bergé (kaballo86_at_[hidden])
Date: 2012-10-13 19:20:38

Hi All,

I'm experimenting with Proto to build a DSEL that operates on geometric 
vectors. I'm trying to write a transform that would take an assign 
expression and unroll it component wise. For instance, I want to replace

     p = q + r;

by

     p[0] = q[0] + r[0],
     p[1] = q[1] + r[1],
     ...,
     p[N] = q[N] + r[N],
     p;

So far I have been able to *mostly* make it work by making a transform 
template unroll_vector_expr that recursively unrolls the expression for 
each vector component, in conjunction with a distribute_subscript 
transform (which I shamelessly took from Eric's article on C++Next).

I don't seem to understand whats the rationale for the 'parameters' used 
with `boost::result_of` and `result_of::make_expr`. The documentation, 
examples and internal code seem to mix `Expr`, `impl::expr` and 
`impl::expr_param`. I'm not sure what should I be using and when so that 
the `result_type` matches the actual result type. Currently I made it 
work by trial an error, by examining the error messages and fixing the 
`const` and `&` discrepancies (code for the transform attached). 
However, as soon as I deep copy the expression the terminals are no 
longer hold by reference and my code fails. Which incidentally brings me 
to my second question...

When deep copying an expression the 'identity' of the terminals is lost, 
as each reference to a terminal results in a new hold by value terminal. 
Consequently, expressions with side effects (like assignment) no longer 
evaluate correctly. Is it possible to customize the expression 
'semantics' so that when a deep copy is made terminals are copied only 
the first time they are seen, and subsequent uses refer to that first 
copy? Or should I be taking a completely different approach, perhaps by 
leaving operations with side effects outside of the grammar?

Thanks in advance for any help, and sorry if any of my questions is 
trivial or erroneous.

Agustín K-ballo Bergé.-
http://fusionfenix.com