On Fri, May 6, 2011 at 11:25 AM, Andrzej <[email protected]> wrote:
> (let ((unquote apply))
> `((unquote + (list 1 2)))) => ((unquote + (list 1 2))) ; plt-scheme
> (R6RS mode) (1)
> (let ((unquote apply))
> `((unquote + (list 1 2)))) => (#<procedure:+> (1 2)) ; isn't it a
> desired output (because of referential transparency)? (2)
> (let ((unquote apply))
> `((unquote + (list 1 2)))) => ((apply + (list 1 2))) ; or this
> (because of macro hygiene)? (3)
>
> After all we can rebind anything else just fine:
> (let ((+ -))
> `((unquote + (list 1 2)))) => (#<procedure:-> (1 2))
>
> > ...which makes the most sense to me.
>
> Can you explain why is this a desirable output? I'm just wondering if
> this is a subtle bug in plt-scheme, my lack of understanding of R6RS
> or an inherent issue with it.
>
> After all the discussion in this thread I'm in favor of (3) (which
> essentially means that UNQUOTE being an identifier). (2) would be the
> behavior with keywords being matched by name. (1) is somewhere in
> between.
Your (3) is close, but the rationalization:
After all we can rebind anything else just fine:
(let ((+ -))
`((unquote + (list 1 2)))) => (#<procedure:-> (1 2))
is unfair, because this example gives UNQUOTE its special meaning (to
QUASIQUOTE) back. A better example would be:
(let ((+ -))
`((not-so-unquote + (list 1 2))))
Can we agree that this does *not* evaluate to '((not-unquote #<procedure:->
(1 2))) ?
In your (3), you I infer you recognize that UNQUOTE has lost its special
meaning to QUASIQUOTE due to the local binding. But the value of the
binding is irrelevant. To QUASIQUOTE it just becomes another unrecognized
identifier to be copied through.
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