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 14:58:28

Eric Niebler wrote:

> On 12/7/2010 12:57 PM, Thomas Heller wrote:
>> 
>> *Dough* misunderstanding here. I didn't mean to clean up the phoenix
>> scope expressions with the help of proto::let. I was thinking, maybe
>> proto::let can borrow something from phoenix scopes on a conceptual
>> level.
> 
> Oh, sure. How does Phoenix handle let scopes? Are local variables
> statically or dynamically scoped? How is it accomplished?

Phoenix in general has an abstract concept of an environment. This 
environment is used to store the arguments of a lambda expression in a 
tuple. This leads to the only requirement this environment must have:
access the arguments with a compile-time index in that tuple, using 
fusion::at_c<Index>(env).
When having a let statement, A new environment is created which acts as a 
map like data structure, indexed by the local names. Additionally the above 
requirements are fulfilled.
Local variables are dynamically scoped. Additionally you can access locals 
defined in some outer let scope.

This is basically how it is working. I was thinking, maybe, proto can adapt 
this concept of an environment. This would allow pretty nifty things, like 
not being tied to only state and data, which really can be a limitation, 
sometimes. Backwards compatibility can be provided by transparently change 
proto::_state and proto::_data to do the right thing.