[racket] Macro defining functions question

On May 15, 2014, at 10:09 AM, Matthias Felleisen <matthias at ccs.neu.edu> wrote:

> 
> On May 15, 2014, at 12:32 PM, Kevin Forchione <lysseus at gmail.com> wrote:
> 
>> This one leaves me scratching my head. Why doesn’t this work? When you run the macro the functions aren’t defined outside of the extent of the begin.
>> 
>> (define-syntax (foo stx)
>> (syntax-case stx ()
>>   [(foo)
>>    #'(begin
>>        (define (bar x) x)
>>        (define (baz x) x))]))
>> 
>> After inning the macro (bar 3) produces an bar undefined error. 
> 
> 
> Racket's macro system is hygienic, which means that by default, the lexical context of the macro usage is kept apart from the macro definition. 
> 
> 
>> On the other hand, this works:
>> 
>> (define-syntax (defstruct-lite stx)
>> (syntax-case stx ()
>>   [(defstruct-lite name field ...)
>>    (let ([make-id
>>           (lambda (template . ids)
>>             (let ([str (apply format template (map syntax->datum ids))])
>>               (datum->syntax stx (string->symbol str))))])
> 
> 
> The above line explicitly transfers the lexical context from the original syntax to the 'made up' make names. Hence these identifiers are now in the same scope as the original syntax (dubbed stx). 
> 
> 
Ok… so apparently the crucial element is the transfer of lexical context to the function id, not necessarily to the arguments or body text? I’m not sure what the rule is for this lexical transfer. For instance, the following defines bar, but not baz:

(require (for-syntax racket/syntax))

(define-syntax (foo stx)
  (syntax-case stx ()
    [(_)
     (with-syntax ([bar (format-id stx "~a" #'bar)]
                   [y (format-id stx "~a" #'y)])
       #'(begin
         (define (bar x) x)
         (define (baz y) y)))]))

Incidentally the macro stepper shows bar in the define differently colored from the rest of the definition, while it shows y in the baz definition as being differently colored. I’m not sure what the color key is though! 

-Kevin