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Subject: Re: [proto] Thoughts on traversing proto expressions and reusing grammar
From: Eric Niebler (eric_at_[hidden])
Date: 2010-10-13 19:58:16
On 10/4/2010 12:20 PM, Thomas Heller wrote:
> On Mon, Oct 4, 2010 at 8:53 PM, joel falcou wrote:
>> On 04/10/10 20:45, Eric Niebler wrote:
>>>
>>> I'm not opposed to such a thing being in Proto, but I
>>> (personally) don't feel a strong need. I'd be more willing if I
>>> saw a more strongly motivating example. I believe Joel Falcou
>>> invented something similar. Joel, what was your use scenario?
>>
>> NT2 ;)
>>
>> More specifically, all our transform are built the same way: visit
>> the tree, dispatch on visitor type + tag and act accordignly. It
>> was needed for us cause the grammar could NOT have been written by
>> hand as we supprot 200+ functions on nt2 terminal. All our code is
>> somethign like "for each node, do Foo" with variable Foo depending
>> on the pass and duplicating the grammar was a no-no.
Fair 'nuf. But I haven't yet reached the conclusion that visitor is the
appropriate solution.
>> We ended up with somethign like this, except without switch_ (which
>> I like btw), so we can easily add new transform on the AST from the
>> external view point of user who didn't have to know much proto. As
>> I only had to define one grammar (the visitor)
This is something I have trouble with. A visitor is a grammar? That
doesn't make sense to me.
>> and only specialisation of the visitor for some tag, it compiled
>> fast and that was what we wanted.
>>
>> Thomas, why not showign the split example ? It's far better than
>> this one and I remember I and Eric weren't able to write it usign
>> grammar/transform back in the day.
Joel, I don't recall this particular problem or being unable to solve it
with Proto transforms.
> The split example was one of the motivating examples, that is
> correct, though it suffers the exact points Eric is criticizing. The
> split example was possible because i added some new transforms which
> proto currently misses, but i didn't want to shoot out all my
> ammunition just yet :P
>
> But since you ask for it:
>
> http://github.com/sithhell/boosties/blob/master/proto/libs/proto/test/splitter.cpp
>
> the new thing i added is transform_expr, which works like
> fusion::transform: It creates the expression again, with transformed
> child nodes (the child nodes get transformed according to the
> transform you specify as template parameter
If the theory is that this particular problem cannot be implemented with
existing Proto grammars and transforms, then I submit the attached
solution as contrary evidence. I'll admit that it doesn't have the
property that the transforms can be substituted post-hoc into an
existing Proto algorithm, but is that really necessary? There is no
grammar to speak of; *every* expression is a valid split expression.
This solution is much, much simpler than Thomas' solution that uses a
visitor. Heck, it took me longer to understand Thomas' code than it did
for me to write the solution using standard Proto components.
Still not seeing the big picture with visitors,
-- Eric Niebler BoostPro Computing http://www.boostpro.com
Proto list run by eric at boostpro.com