Proto for Boost mailing list page: Re: [proto] Manipulating an expression tree

Subject: Re: [proto] Manipulating an expression tree
From: Bart Janssens (bart.janssens_at_[hidden])
Date: 2011-04-06 16:53:21

On Wed, Apr 6, 2011 at 10:29 PM, Karsten Ahnert
<karsten.ahnert_at_[hidden]> 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,

-- 
Bart