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

Subject: Re: [proto] restructuring expression
From: Bart Janssens (bart.janssens_at_[hidden])
Date: 2012-05-29 16:33:19

On Tue, May 29, 2012 at 8:21 PM, Eric Niebler <eric_at_[hidden]> 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.

Just thinking out loud, could something like this work and be made recursive:

boost::proto::when
<
    proto::multiplies< <grammar for anything except double>,
proto::terminal<double> >,
    proto::functional::make_multiplies(proto::_right, proto::_left)
>

?

Cheers,

-- 
Bart