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

Thanks!


On Fri, May 9, 2014 at 11:33 AM, David Van Horn <dvanhorn at cs.umd.edu> wrote:

> 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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> >>
> >
> >
> >
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> >
>
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