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.
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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