Well, he's generous about it; here's what he had to say. John
Begin forwarded message: > From: Aaron Stump <aaron-st...@uiowa.edu> > Date: November 16, 2010 5:58:42 PM PST > To: John Clements <cleme...@brinckerhoff.org> > Subject: Re: Q. about "Directly Reflective" paper > Reply-To: ast...@cs.uiowa.edu > > Hi, John. > > I think you are right about this. Lambda abstractions evaluate to > #procedures in Scheme R5RS, and so it is not possible to take a cdr or car of > one of these. I have no idea why I wrote this (four years ago -- there was a > major lag between acceptance and publication at HOSC). I will add a note to > my web page about this, and possibly upload a revised version of the paper > without this incorrect statement. > > Aaron > > On Tue, Nov 16, 2010 at 4:18 PM, John Clements <cleme...@brinckerhoff.org> > wrote: > I'm reading your paper, "Directly Reflective Meta-Programming," and I got > stuck early on a remark of yours about Scheme: > > > A meta-programming language is scope safe (or hygienic) iff variables may > > not be captured or escape their scopes during computation. Dynamic > > variables in Emacs LISP and Common LISP are a good example of a violation > > of scope safety [30, 24]. Scheme R5RS’s macro language is designed to be > > scope safe [21]. Other constructs in Scheme R5RS, however, enable violation > > of scope safety, even though the language does not have dynamic variables. > > For a violation of scope safety in spirit, though not technically, we have > > that (caddr ’(lambda (x) x)) evaluates to x. According to the R5RS language > > definition, ’(lambda (x) x) is a literal expression, and hence the > > occurrences of x in it are not variables at all, but just (unscoped) > > literal data. So in this example, a variable has been created (namely, the > > resulting unquoted x), but not by means of removing it from its scope. > > Using quasiquotation, however, the example may be modified to give a true > > violation of scope safety. The following expression extracts the variable x > > from its scope, by transforming the binding lambda expression into a piece > > of literal data, and then extracting and evaluating the quoted variable. > > > ((lambda (y) (eval ‘(car (cdr (cdr ’,y))))) (lambda (x) x)) > > > This looks pretty goofy to me. Do you know of R5RS implementations that > actually allow you to peel apart a 3d value like this? Racket (nee MzScheme) > certainly doesn't. > > Thanks! > > > John Clements > >
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