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

Subject: Re: [proto] : Proto transform with state
From: Eric Niebler (eric_at_[hidden])
Date: 2010-12-06 14:15:53

On 11/18/2010 3:31 PM, Eric Niebler wrote:
> On 11/18/2010 1:45 PM, Thomas Heller wrote:
>> Eric Niebler <eric_at_...> writes:
>>> It's REALLY hard. The let context needs to be bundled with the Expr,
>>> State, or Data parameters somehow, but in a way that's transparent. I
>>> don't actually know if it's possible.
>>
>> Very hard ... yeah. I am thinking that we can maybe save these variables in the 
>> transform?
> 
> I'm thinking we just stuff it into the Data parameter. We have a
> let_scope template that is effectively a pair containing:
> 
> 1. The user's original Data, and
> 2. A Fusion map from local variables (_a) to values.
> 
> The let transform evaluates the bindings and stores the result in the
> let_scope's Fusion map alongside the user's Data. We pass the let_scope
> as the new Data parameter. _a is itself a transform that looks up the
> value in Data's Fusion map. The proto::_data transform is changed to be
> aware of let_scope and return only the original user's Data. This can
> work. We also need to be sure not to break the new
> proto::external_transform.
> 
> The problems with this approach as I see it:
> 
> 1. It's not completely transparent. Custom primitive transforms will see
> that the Data parameter has been monkeyed with.
> 
> 2. Local variables like _a are not lexically scoped. They are, in fact,
> dynamically scoped. That is, you can access _a outside of a let<>
> clause, as long as you've been called from within a let clause.
> 
> Might be worth it. But as there's no pressing need, I'm content to let
> this simmer. Maybe we can think of something better.

I played with the let transform idea over the weekend. It *may* be
possible to accomplish without the two problems I described above. See
the attached let transform (needs latest Proto trunk). I'm also
attaching the Renumber example, reworked to use let.

This code is NOT ready for prime time. I'm not convinced it behaves
sensibly in all cases. I'm only posting it as a curiosity. You're insane
if you use this in production code. Etc, etc.

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