Re: [boost] [proto] transform for flattening an expression - how?

From: Larry Evans (cppljevans_at_[hidden])
Date: 2008-03-17 18:21:25

On 03/17/08 16:35, Markus Werle wrote:
> Hi!
> 
> Sorry, Eric, if this is a lot of work.
> Reject to do it, if you like, or provide only 
> some of these.
> 
> ---- Task 1 --------------
> Consider I need to flatten a tree into a sum:
> 
> template<int I> struct arg {};
> 
> proto::terminal<arg<1> >::type a;
> proto::terminal<arg<2> >::type b;
> proto::terminal<arg<3> >::type c;
> proto::terminal<arg<4> >::type d;
> 
> a + b + c + d 
> 
> has type expr<tag::plus, arg2<a tree structure> >
> 
> but I need a template function make_flat
> taking arbitray expressions and return 
> expressions of type
> 
> expr<sum, args4<all args in the typelist> >
> 
> How can I do this in proto?
> 
> -----------
> Task 2: Next step:
> 
> a - b + c should first be transformed into
> a + (-b) + c and then transformed into the sum.
> 
> Could you please provide full compilable examples?
> 
> ------
> 
> Task 3: Next step:
> 
> I want to detect simultaneous occurences of
> x and -x and have them dropped from the typelist of my sum.
> Please provide an example that detects this for terminals only,
> then one which also works for (a*b - a*b) and returns
> expr<tag::nil, nulllist> (BTW, is there a nil/null tag in proto?)
> 
> 
> ---------
> 
> Task 4: Write an expand function that for
> a*(b+c) returns a type representation a*b+a*c
> 
> -----
> 
Me too!  In general, there should be an n-ary version of all the
binary ops.  This should also be extended to the or_ also.
I remember having a problem with folding or_ because it was
an n-ary op and had to define my own binary or which just
forwarded to the or_.  That was a "kludge".  So maybe
define or_ as binary and then define the n-ary version
based on this binary.