Proto for Boost mailing list page: Re: [proto] : Proto transform with state

Subject: Re: [proto] : Proto transform with state
From: Thomas Heller (thom.heller_at_[hidden])
Date: 2010-12-07 15:40:38

Eric Niebler wrote:

> On 12/7/2010 3:13 PM, Thomas Heller wrote:
>> Eric Niebler wrote:
>>> Now they do: T()(e,s,d). Inside T::impl, D had better be the type of d.
>>> Nowhere does the _data transform appear in this code, so changing _data
>>> to be "smart" about environments and scopes won't save you if you've
>>> monkeyed with the data parameter.
>> 
>> Very true. Something like proto::transform_env_impl could help. Introduce
>> a new type of primitive transform which is aware of this environment. The
>> usual transform_impl can still be used.
>> By calling T()(e,s,d) you just create a 2-tuple. The first parameter is
>> the state, second data.
>> Just thinking out loud here ...
> 
> So transform_impl strips the data parameter of the let scope stuff, and
> the local variables like _a don't use transform_impl and see the scope
> with the locals?
> 
> Well, it's not that simple.... Consider:
> 
>   make< int(_a) >
> 
> If make::impl uses transform_impl, it strips the let scope before _a
> gets to see the local variables. If this is to work, most of Proto's
> transforms must be "special" and pass the let scope through. That means
> proto::call, proto::make, proto::lazy and proto::when, at least. But ...
> it just might work. Good thinking!
> 
> <time passes...>
> 
> No, wait a minute. Look again:
> 
> struct T : transform<T> {
> 
>   template<class E, class S, class D>
>   struct impl : transform_impl<E, S, D> {
> 
>     // do something with D
> 
>   };
> 
> };
> 
> T::impl gets passed D *before* transform_impl has a change to fix it up.
> Any existing transform that actually uses D and assumes it's what they
> passed in is going to be totally hosed. In order to make existing
> transforms let-friendly, they would all need to be changed. That's no
> good.
> 
> Bummer, I was excited for a minute there. :-/

Yes ... this would basically mean a complete rewrite ... no good.
--> Proto V5, you might save the thought for the C++0x rewrite :)