[racket] Q. about 'nostops' in MTWT paper

On 5/9/14, 11:29 AM, Alexander McLin wrote:
> Do you mind expanding on what MTWT stands for? I'm not familiar with this
> paper and am curious.

Macros that Work Together
http://www.eecs.northwestern.edu/~robby/pubs/papers/jfp2012-fcdf.pdf


> On Thu, May 8, 2014 at 6:20 PM, John Clements <clements at brinckerhoff.org>wrote:
> 
>>
>> On May 8, 2014, at 3:00 PM, Ryan Culpepper <ryanc at ccs.neu.edu> wrote:
>>
>>> IIRC, the typesetting of environment extension might suggest replacement
>> in a mapping, but I believe it actually means 'cons' in an association
>> list, and 'nostops' is one thing that relies on the latter implementation
>> of the environment rather than the former.
>>
>> Ah… so the model does in fact have the “uncovering” behavior I expected it
>> to.  In this case, I think that the text of the paper is mistaken; it says
>>
>> nostops[xi] = {var -> transform | xi(var) = transform and transform !=
>> STOP }
>>
>> which certainly suggests to me that these mappings simply wouldn’t appear
>> in the updated mapping.
>>
>> Amazing how ambiguous formal syntax can be.
>>
>> It seems like the right way to fix this would be to explicitly separate
>> the environment xi into a mapping and a stop list—the ‘nostops’ definition
>> would be trivial, but lookup in xi would get slightly uglier. Perhaps a
>> footnote somewhere in the paper?
>>
>> Anyway, thanks for clarifying.
>>
>>
>>
>> John
>>
>>>
>>> So an environment with 'lambda' in the stop list actually looks like
>>>
>>>  ((lambda STOP) (lambda FUN))
>>>
>>> so when 'nostops' filters it, it produces
>>>
>>>  ((lambda FUN))
>>>
>>> and the previous meaning of 'lambda' is restored.
>>>
>>> Ryan
>>>
>>>
>>> On 05/08/2014 04:58 PM, John Clements wrote:
>>>> I’m reading through MTWT for the nth time, and this time I have a
>> question about the ‘nostops’ metafunction, part of local expansion.
>>>>
>>>> Specifically: ‘nostops’ takes an environment xi, and strips out
>> everything bound to 'STOP’. This prevents the ’STOP’s from an enclosing
>> local expand from being in force in this one.  However, it seems to me that
>> the ’nostops’ metafunction should restore the bindings that were in place
>> before they were mapped to STOP. More specifically; suppose a local-expand
>> specifies, say, ‘lambda’ as a STOP. This replaces the mapping from ‘lambda’
>> to FUN with a mapping from ‘lambda’ to ’STOP’. Then, a nested local-expand
>> uses a different stop-list, that doesn’t include ‘lambda’. If I’m reading
>> the paper correctly, this local expansion would then take place in a ‘xi’
>> where there was no binding for lambda at *all*, which would presumably lead
>> to an error during that expansion.
>>>>
>>>> I checked out the model, to see if this was a typesetting issue, but it
>> seems pretty clear to me that the model linked to by the paper has exactly
>> this property; it’s simply filtering the ‘xi’ environment to throw out
>> things that were bound to STOP.
>>>>
>>>> I tried to construct a racket program to illustrate my question, and
>> here’s what I came up with:
>>>>
>>>> #lang racket
>>>>
>>>> (let-syntax ([boring (lambda (stx) (cadr (syntax->list stx)))])
>>>>
>>>>   (let-syntax ([a (lambda (stx)
>>>>                     (local-expand (cadr (syntax->list stx))
>>>>                                   'expression
>>>>                                   (list #'boring)))])
>>>>     (let-syntax ([b (lambda (stx)
>>>>                       (local-expand (cadr (syntax->list stx))
>>>>                                     'expression
>>>>                                     (list)))])
>>>>       (a (b (boring 34))))))
>>>>
>>>> Unsurprisingly, this evaluated to the “correct” value, 34.  This means
>> that either the model doesn’t match Racket, or (more likely) I’m
>> misunderstanding the model. Or possibl
>>>>
>>>> Ah, what the heck; I’ll cc: the whole racket list. I’ll take help
>> wherever I can get it :).
>>>>
>>>> John
>>>>
>>>
>>
>>
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