Hey Ramana,
On Sat, 20 Jun 2009 03:46:51 -0400, Ramana Kumar <[email protected]>
wrote:
> How would you write bound-identifier? in R6RS? (A function that takes
> an identifier and returns a boolean indicating whether that identifier
> is bound.)
A quick read over the R6RS stuff indicates that it is similar and/or the
same as the Chez implementation of 'free-identifier=?', which I think is
sufficient in this case to make the 'bound-identifier?' form you desire.
> I'm interested in
> 1. A clearly portable and clean definition in R6RS, if it exists, or
> 2. Any reasons why including a primitive bound-identifier? (in the
> syntax-case library, say) would be a bad idea (presuming there is no
> solution to 1).
I do not know if this is really clearly portable, but to me it is not
obviously importable either. Since this can be done at any point in
general outside of the top-level with 'free-identifier=?' I don't see a
real reason to put another predicate in there.
(define-syntax bound-identifier?
(lambda (x)
(syntax-case x ()
[(k y) (identifier? #'y)
#`#,(not (free-identifier=? #'y
(datum->syntax #'a (syntax->datum #'y))))])))
I think the choice for #'a may be important, but if this form is defined
at the top level and 'a' is undefined, this should work. I think it works
regardless so long as this form is defined at the top level. This form
will only find bound identifiers in lexical scopes, since everything is
free-identifier=? when we reach the top level in this case. However, I
think this should work pretty well.
Sincerely,
Aaron W. Hsu
--
Of all tyrannies, a tyranny sincerely exercised for the good of its
victims may be the most oppressive. -- C. S. Lewis
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