[plt-scheme] Problem with (... ...) pattern

Matthew Flatt skrev:
> At Thu, 15 Nov 2007 13:22:01 +0100, Jens Axel Soegaard wrote:
>> I tried this definition:
>>
>>    (define (formals-arity stx)
>>      (syntax-case stx ()
>>        [(a ... (... ...))
>>         (- (length (syntax->list #'(a ... ))))]
>>        [(a ...)
>>         (length (syntax->list stx))]
>>        [else
>>         (error 'formals-arity "not-yet implemented arity for " stx)]))
>>
>> Alas, I get the error:
>>
>>      syntax: missing ellipses with pattern variable in template in: a
>>
>> and the a in #'(a ...) is colored red.
> 
> Tricky!
> 
> The `(... ...)' in the pattern means that `...' gets bound as a pattern
> variable. In the scope of that binding, `(a ...)' as a template is the
> `a' pattern variable followed by the `...' pattern variable.

Ah! Thanks for the explanation.

-- 
Jens Axel Søgaard