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

Subject: Re: [proto] restructuring expression
From: Eric Niebler (eric_at_[hidden])
Date: 2012-05-29 14:21:32

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.

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com