(Please don't top-post. Rearranging...)
On 4/7/2011 5:45 AM, Karsten Ahnert wrote:
> On 04/06/2011 10:53 PM, Bart Janssens wrote:
>> On Wed, Apr 6, 2011 at 10:29 PM, Karsten Ahnert
>> <karsten.ahn...@ambrosys.de> wrote:
>>> Is there a direct way to transform an expression tree into another one?
>>> For example, is it possible that every proto::plus< > node is
>>> transformed to it left child? I tried to solve this problem via protos
>>> build-in transforms without success. It seems that they are suited for
>>> evaluation of an existing tree, but I might be wrong.
>>
>> Hi Karsten,
>>
>> I'm pretty sure they can do both. For your example, I think something
>> along the lines of this might work (untested):
>>
>> struct LeftPlus :
>> boost::proto::or_
>> <
>> boost::proto::terminal<boost::proto::_>,
>> boost::proto::when
>> <
>> boost::proto::plus<boost::proto::_, boost::proto::_>,
>> LeftPlus(boost::proto::_left)
>> >,
>> boost::proto::nary_expr< boost::proto::_, boost::proto::vararg<LeftPlus>
>> >
>> >
>> {};
>>
>> This should recurse through expressions and replace sequences of
>> pluses with the left-most terminal. You may need some other criteria
>> to end the recursion depending on your use case.
>>
>> Disclaimer: I'm relatively new to proto myself, so the experts might
>> have better solutions!
>>
>> Cheers,
>
> Great! It works perfectly, alltough I don't understand the code
> completely yet.It takes time, but it'll be worth it. You can't do much with Proto with grokking grammars and transforms. > Another question is: can a node have a state. In my algorithm it would > be nice, if every proto::multiplies< > node stores some intermediate > values which are used during later evaluations of the tree. No. Intermediate expression nodes carry no run-time state in their nodes. They only carry compile-time information in the form of the tag. That's how a plus node is distinguished from a minus node, for instance. If you need to compute intermediate values, you can use a transform to build a parallel structure. -- Eric Niebler BoostPro Computing http://www.boostpro.com
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