Re: [Boost-users] [Proto] implementing an computer algebra system with proto

Subject: Re: [Boost-users] [Proto] implementing an computer algebra system with proto
From: Eric Niebler (eric_at_[hidden])
Date: 2009-02-09 18:33:08

Kim Kuen Tang wrote:
> Hi Dave and Eric,
> 
> i have written an grammar to do some transformations on  an expression 
> and would like to know your opinions.
> 
> If the grammar matches the pattern var_+a, then it should transform the 
> tree to var_=(0-a)/1.
> So the resulting callable transform would be :
> 
> proto::when<
>            //var_+a
>            proto::plus<proto::terminal<placeholder>, 
> proto::terminal<proto::_> >
>            ,proto::_make_assign(
>                proto::_left
>                ,proto::_make_divides(proto::_make_minus(Zero 
> ,proto::_right),One )
>            )
> 
> How should one implement the two macros Zero and One.

Try this:

// default-constructs an int
struct Zero
   : proto::make<int>
{};

// initializes and int with a default-constructed mpl::int_<1>
// object (which is convertible to an int with value of 1).
struct One
   : proto::make<int(mpl::int_<1>())>
{};

HTH,

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