[plt-scheme] datum->syntax rules

The "_" in "(_ test then else)" is a special pattern that match's 
anything. So the #'_ given to datum->syntax is not binding to "if-it". 
This is a PLT v4 change. I think you want:

(define-syntax (if-it stx)
  (syntax-case stx ()
    ((if-it test then else)
     (with-syntax ((it
                    (datum->syntax #'if-it
                                   'it)))
       #'(let ((it test)) 
           (if it then else)
           )))))

Chongkai



YC wrote:
> Hi -
>
> are there specific rules that governs the choice of context to 
> datum->syntax?  I'm finding that the choices of the context impacts 
> whether the name can be captured, and are not seeing what the issues 
> are...
>
> Below are two versions of `if-it` - the first version works with #'_, 
> but the second version cannot work with it (it works with #'test).
>
> Any pointers are appreciated, thanks.
> yc
>
> (define-syntax (if-it stx)
>   (syntax-case stx ()
>     ((_ test then else)
>      (with-syntax ((it
>                     (datum->syntax #'_  
>                                    'it)))
>        #'(let ((it test))
>            (if it test else))))))
>
> (if-it '(abc def) it 'nil) ;; works
>
> (define-syntax (if-it1 stx)
>   (syntax-case stx ()
>     ((_ test then else)
>      (with-syntax ((it
>                     (datum->syntax #'_ ;; doesn't work... works if 
> replaced with #'test
>                                    'it)))
>        #'(let ((it test))  
>            (if it then else)
>            )))))
>
> (if-it1 '(abc def) it 'nil) ;; => reference to undefined identifier: it
>
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