#lang racket

(require (for-syntax syntax/parse))

(begin-for-syntax
  (define-struct object (colour size))
  (define ball (object 'red 'small))

  (define-syntax-class struct
    [pattern s #:fail-unless (struct? (syntax-e #'s))
             "Expected struct?"]))

(define-syntax (touch-struct stx)
  (syntax-parse stx
    [(_ obj:struct)
     #`#,(object-colour #'obj)]))

(define-syntax (run stx)
  (syntax-parse stx
    [(_)
     #'(touch-struct ball)]))

(run) ;why does this fail?