Re: [boost] boost::proto pattern matching and transforms

From: Maurizio Vitale (mav_at_[hidden])
Date: 2007-04-27 05:38:57

Eric Niebler <eric_at_[hidden]> writes:

>> For instance, given the pattern:
>> 
>>  proto::terminal< number<bounds<run_time,_>,_,_,_,_,_> >
>> 
>> how can I refer in the rhs of a transform to:
>
>
> What do you mean by "rhs"?

if you see a transform as pattern -> transformed tree, then pattern would be
the lhs and the transformed tree would be the rhs.

>>     a) the entire pattern being matched (e.g. terminal<> or number<>)
>
>
> Transforms are always of the following form:
>
> template<typename Grammar>
> struct some_transform
>    : Grammar
> {
>    template<typename Expr, typename State, typename Visitor>
>    struct apply
>    {
>      typedef ... type;
>    };
>
>    template<typename Expr, typename State, typename Visitor>
>    static typename apply<Expr, State, Visitor>::type
>    call(Expr const &expr, State const &state, Visitor &visitor)
>    {
>      return ...;
>    }
> };

Oh. Yes, makes sense. Now I got it.
What I was trying was to use the default transform proto::always<pattern, replacement>
hoping that somehow I would have gotten mpl::placeholders bound inside replacement.

> There are no mpl placeholders here, and the placeholders are not 
> positional like that. You can't say, find me what matched the Nth 
> occurance of proto::_. You only get the whole expression that matched.

Would have been nice, though...

>> This is blocking me at the moment, so if anybody has suggestions, they would be very much
>> appreciated.
>
> HTH,

Hugely. Obviously I'll find problems as soon as I start implementing it, but I think
now I've got it.

Thanks again,

       Maurizio