[racket] Getting context info from ellipses pattern vars when calling DATUM->SYNTAX

On Sun, Nov 14, 2010 at 9:57 PM, Jon Rafkind <rafkind at cs.utah.edu> wrote:
> On 11/14/2010 07:52 PM, Sam Tobin-Hochstadt wrote:
>> On Sat, Nov 13, 2010 at 5:09 PM, D Marshall <dmarshall207 at gmail.com> wrote:
>>> I understand that DATUM->SYNTAX gets it's context
>>> information from it's first argument.  I have no problems
>>> when using simple (non-ellipses) pattern variables, however,
>>> I haven't been unable to get a strictly "ellipses pattern"
>>> to work in this context (no pun intended).
>> Here are two options:
>>
>> This is my preference - use the context of the macro application
>>     (define-syntax test-this
>>       (lambda (x)
>>         (syntax-case x ()
>>           [(_ a ...)
>>              (with-syntax
>>               ([form    (datum->syntax x
>>                            `(+ ,@(syntax->datum (syntax (a ...)))))])
>>               (syntax form))])))
>>
>> This is also common, but can break in some cases:
>>     (define-syntax test-this
>>       (lambda (x)
>>         (syntax-case x ()
>>           [(kw a ...)
>>              (with-syntax
>>               ([form    (datum->syntax (syntax kw)
>>                            `(+ ,@(syntax->datum (syntax (a ...)))))])
>>               (syntax form))])))
>>
>> The reason what you tried to do didn't work is that you need a context
>> that is from the context of the macro use.  The rest of the list isn't
>> something the programmer originally wrote, but something that was
>> generated by your macro, so it doesn't have any useful context.
> This works too:
>
> (define-syntax test-this
>  (lambda (x)
>    (syntax-case x ()
>      [(_ a ...)
>       (with-syntax
>         ([form (datum->syntax (car (syntax->list #'(a ...)))
>                               `(+ ,@(syntax->datum #'(a ...))))])
>         (syntax form))])))

Well, sometimes:

> (test-this)
car: expects argument of type <pair>; given '()
-- 
sam th
samth at ccs.neu.edu