Proto for Boost mailing list page: Re: [proto] restructuring expression

Subject: Re: [proto] restructuring expression
From: Joel Falcou (joel.falcou_at_[hidden])
Date: 2012-05-30 07:33:55

On 05/29/2012 08:21 PM, Eric Niebler wrote:
> On 5/29/2012 1:44 AM, Karsten Ahnert wrote:
>> I have an arithmetic expression template where multiplication is
>> commutative. Is there an easy way to order a chain of multiplications
>> such that terminals with values (like proto::terminal<  double>) appear
>> at the beginning? For example that
>>
>> arg1 * arg1 * 1.5 * arg1
>>
>> will be transformed to
>>
>> 1.5 * arg1 * arg1 * arg1
>>
>> ?
>>
>> I can imagine some complicated algorithms swapping expressions and child
>> expressions but I wonder if there is a simpler way.
> There is no clever built-in Proto algorithm for commutative
> transformations like this, I'm afraid. I was going to suggest flattening
> to a fusion vector and using fusion sort, but I see there is no fusion
> sort! :-( Nevertheless, that seems like a promising direction to me.
> Once you have the sorted vector, you should(?) be able to use
> fusion::fold to build the correct proto tree from it.
>

Won't having a way to build it properly from the get go be a better 
solution ?

This basically require the feature we spoke about earlier so that 
building a X * Y
node check which from X or Y is a double and put it in the proper place ?

Then when doing X * Expr, check if there is a double at child<0> of expr
and restructure the whole tree at generation time ?