[r6rs-discuss] [Formal] SRFI-39 should be made an R6RS library

[Felix Klock]

On Feb 21, 2007, at 1:42 AM, John Cowan wrote:

> Felix Klock scripsit:
>
>> Am I right in thinking that in all of the sample implementations of
>> parameterize that have been presented so far, the body of the
>> parameterize expression is not in tail position with respect to the
>> entire expression?
>>
>> I do not know what Scheme implementations besides PLT Scheme support
>> this, but it is one of the potential properties of parameterize that
>> I think we should be discussing.
>
> Is this done using the Bacon/Baker technique?
>
> http://www.accesscom.com/~darius/writings/dynatail.html
> http://home.pipeline.com/~hbaker1/BuriedStale.html


The way PLT Scheme does it is via "continuation marks."  Other people  
on this list can do a better job than I explaining the details, but  
the quick version of the idea is that the continuation structure is  
extended to carry particular values (or more generally, key/value  
pairs) and updates to these values are done in a fashion that  
preserves tail recursion.

 From my preliminary skimming of the links you gave, it seems like  
continuation marks might be considered a useful generalization of the  
ideas presented there:
1. Continuation marks allow different modules to install continuation  
marks in a safe-for-space fashion without the two modules being aware  
of each other.  (Even when they each call each other tail recursively.)
2. Continuation marks provide a controlled interface to the sort of  
stack inspection described by Bacon.  There's no need to hack on the  
interpreter: they're already there, and you get to install your own  
marks in your code.

You can find examples illustrating these advantages in John Clements'  
work (as well as motivating examples for providing continuation marks  
in the first place.)

On the other hand, Clements' thesis does not cite Bacon or Baker.  I  
infer that either John was not aware of their work, or there is some  
key distinction differentiating PLT's continuation marks from the  
Bacon/Baker approach.

Anyway, I am not suggesting that we add continuation marks to R6RS.   
I mainly wanted to point out that:
1. There are other approaches to implementing parameterize besides  
using dynamic-wind
2. These other approaches may have positive characteristics (such as  
improved asymptotic space efficiency)
3. If R6RS standardizes on an interface for parameters,  it should  
not rule out this sort of implementation for parameterize.  (I do not  
know if any of the proposed parameter interfaces would do so.)

-Felix