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

Do you mind expanding on what MTWT stands for? I'm not familiar with this
paper and am curious.


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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