Re: [Boost-users] [proto] need explanation in implementing distributive law

Subject: Re: [Boost-users] [proto] need explanation in implementing distributive law
From: Eric Niebler (eric_at_[hidden])
Date: 2010-10-05 16:17:22

On 10/5/2010 5:43 AM, Mauro Bianco wrote:
> Hi all,
> It is probably already been discovered by everyone at this point, but
> the code proposed by Andrew does not to work properly to apply
> distributivity more than once in an expression [works for a*(b+c) but
> not for a*(b+c*(d+c)) ]

Funny, I never saw the original message. Perhaps it's stuck in a
moderation queue or lost in the ether(net).

> Here a modified version that seems to work recursively.
> How does it look?
> 
> struct distr :
>   proto::or_<
>     proto::when<
>       proto::multiplies< proto::_, proto::plus< proto::_, proto::_ > >,
>       proto::call<distr( // THIS IS TO RECURSE
>         proto::_make_plus(
>           proto::_make_multiplies(
>             proto::_left,
>             proto::_left( proto::_right )
>           ),
>           proto::_make_multiplies(
>             proto::_left,
>             proto::_right( proto::_right )
>           )
>         )
>       )
>     >
>   >,
>   proto::otherwise<proto::nary_expr<proto::_, proto::vararg<distr> > > >
> {};

Yes, this looks reasonable.

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