> On Sep 4, 2015, at 4:26 AM, Konrad Hinsen <[email protected]> wrote:
>
> Brian Mastenbrook writes:
>
>> It's a capture problem. In the first case, you're just binding the
>> name "send" locally and all is well. In the second case, you're
>> trying to introduce a binding for "send" that you didn't get from
>> the input form.
>
> Ahhh.... that one has bitten me before, but I had forgotten about it.
> Thanks for the reminder!
>
>> You're also getting a confusing error because "send" is already
>> bound; try using a name that's not already defined and you should
>> get an unbound identifier.
>
> I used a bound symbol intentionally to see if I can shadow an existing
> syntax transformer binding.
Ok, if you *really* want to shadow an existing binding, you can use
datum->syntax, but using a syntax parameter is much better:
(define-syntax (def stx)
(syntax-parse stx
[(_ (fn-name:id arg:id ...) body ... )
#:with send-id (datum->syntax #'fn-name 'send)
#'(define (fn-name arg ...)
(let-syntax ([send-id (λ (stx)
(syntax-parse stx
[(send-id obj:expr method:id x:expr (...
...))
#'(method obj x (... ...))]))])
body ...))]))
>> Alternatively, you can use a syntax parameter, which is probably
>> the ideal solution here.
>
> At least it's one that works. It feels like cheating to use dynamic
> scoping to get around a problem with lexical scoping, but knowing when
> to cheat is a fundamental competence when dealing with any bureaucracy ;-)
Um, the reason a syntax parameter is better is that it *does* follow the
lexical scoping rules, where the unhygienic version does weird things you
wouldn't expect. Using datum->syntax is cheating a lot more, and syntax
parameters deal with problems like this in a much better way.
P.S.
Have you read Fear of Macros? If you haven't, I highly recommend it because
it's awesome.
> Thanks again,
> Konrad.
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