[plt-scheme] Re: misplaced ellipses

On Feb 26, 2008, at 3:41 PM, David Van Horn wrote:

> Matthew Flatt wrote:
>> At Tue, 26 Feb 2008 13:48:45 -0500, David Van Horn wrote:
>>> (define-syntax f
>>>    (syntax-rules ()
>>>      [(f id)
>>>       (syntax (... id))]))
>>>
>>> (f ...)
>>> ;; expect (syntax (... ...)),
>>> ;; ie., the identifier `...'.
>>>
>>>
>>> I get instead:
>>>
>>> syntax: misplaced ellipses in template in: ...
>>>
>>> Why is this the case?
>> The macro `(f ...)' expands to `(syntax ...)', which is indeed a 
>> syntax
>> error.
>
> If the syntax-rule is
>
>   (f id) =transcribe=> (syntax (... id))
>
> I would expect (f ...) =transcribe> (syntax (... ...)), which is not a 
> syntax error.
>
>> I think you meant
>> (define-syntax f
>>    (syntax-rules ()
>>      [(f id)
>>       (... id)]))     ; no `syntax'
>
> No, I want a macro that takes an identifier, expands into an 
> expression that when evaluated, produces the syntax object 
> representing the identifier (even when this identifier is the ... 
> one).
>
> So for example (identifier? (f x)) is true.

Matthew already explained how to do ellipsis-escaping with 'syntax', 
but another good tool to be aware of is 'quote-syntax':

(define-syntax f
   (syntax-rules ()
     [(f id)
      (quote-syntax id)]))

Ryan

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