It comes from the way syntax properties are propagated through macro transformers. There is some explanation of this in the docs; search for syntax-property and scroll up.
Robby On Sun, Mar 25, 2012 at 7:05 PM, Danny Yoo <[email protected]> wrote: > I'm getting confused by some behavior with regards to paren-shape. > Here's what I see: > > #lang racket > > ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; > (define-for-syntax (square-brackets? stx) > (eq? (syntax-property stx 'paren-shape) #\[)) > > (define-syntax (square-app stx) > (syntax-case stx () > [(_ op ...) > (square-brackets? stx) > (syntax-property (syntax/loc stx (app op ...)) > 'paren-shape > #\[)])) > > (define-syntax (app stx) > (syntax-case stx () > [(_ op ...) > (square-brackets? stx) > #'(#%app op ...)] > [else > (raise-syntax-error #f > (format "expected square brackets. Paren shape is: > ~s" > (syntax-property stx 'paren-shape)) > stx)])) > > [square-app + 3 4] > ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; > > When I run this, I see the following error message: > > app: expected square brackets. Paren shape is: (#\[ . #\[) in: (app + 3 4) > > > I do not understand why the paren-shape is a cons pair. Where did > that come from? > > > Thanks! > _________________________ > Racket Developers list: > http://lists.racket-lang.org/dev _________________________ Racket Developers list: http://lists.racket-lang.org/dev
