Re: Error context info

[Matthew Flatt <mflatt@cs.utah.edu>]

Quoting Greg Pettyjohn:
> I tried doing a ... (syntax/loc stx (make-a ...))...  but in this case, no
> line was highlighted.

`syntax/loc' only assigns a location to the top-most syntax object. In
this case, I think you really want to put more specific error handling
in `validate-b':

 (define-syntax validate-b
  (lambda (stx)
    (syntax-case stx (b)
      [(_ (b x ...)) (syntax (make-b (list (validate-c x) ...)))]
      [(_ bad) (raise-syntax-error #f "not a b" (syntax bad))])))

Another possibility would be to give each `(validate-b x)' the location
of its `x'. The following `validate' does that:

 (define-syntax validate
   (lambda (stx)
     (syntax-case stx (a)
       [(_ (a x ...)) 
        (with-syntax ([(valid-x ...)
                       (map
                        (lambda (var)
                          (with-syntax ([x var])
                            (syntax/loc var (validate-b x))))
                        (syntax->list (syntax (x ...))))])
          (syntax/loc stx (make-a (list valid-x ...))))])))

Matthew