Re: [Boost-users] Proto transform using visitor

From: Joel Falcou (joel.falcou_at_[hidden])
Date: 2008-06-04 11:07:21

Eric Niebler a écrit :
> In the proto/v4 branch, "Visitor" has been renamed "Data" to more 
> accurately reflect its role as a blob of user-specified data to be 
> passed along to each transform.
OK so my first intuition was right then ;)

> A very simple example is a lambda library with placeholders. Imagine you 
> wrote a transform that evaluated lambdas with arguments. When evaluating 
> a lambda expression such as (_1 + _2) with the arguments (3,4), you 
> might pass the arguments to the transform as a tuple in the Data 
> parameter (a.k.a Visitor). Then, when the transform has recursed to a 
> placeholder terminal, say _1, the result would be the 0th element in the 
> Data tuple.

Doesn't this features make context obsolete ?
I mean instead of using context to traverse the AST and performs 
evalmuations, oen can simply build a proper visitor and traverse the AST 
in a way simialr to a classic visitor pattern ?

> I have reimplemented the Boost Lambda Library on top of proto using this 
> technique. You can find it here:

I'll get a look. Seems alos that I missed the v4 branches.
Is there large changes between this and the last proto zip that was 
available ont he vault ?

Two other questions :
- why does trasnform apply deep copy onto terminals ?
- I'm currently using proto to implement some parallel programming 
library and I have to explain what I do with proto to people more used 
to the classic compilation/semantic vocabulary than to the proto idioms.
in this context, is it correct to say that transforms are "semantic 
actions" applied onto the element of the AST ?