[racket] Reading Graph Structure: read: #..= expressions not allowed in read-syntax mode (??)

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Hi Matthias,

On 06/06/11 16:58, Matthias Felleisen wrote:
> 
> Do you want something like this: 
> 
> #lang racket
> 
> (struct dl (left node right) #:transparent #:mutable)
> 
> (shared ((middle (dl left   1 right))
>          (left   (dl #false 0 middle))
>          (right  (dl middle 2 #false)))
>   middle)

Ah, yes, thank you, that's what I was looking for.

`shared' seems to depend critically on `dl' being mutable and fails
silently when it isn't. For example:


#lang racket

;;; link structure
(struct _dl (left val right) #:mutable
  #:property prop:custom-write
  (lambda (dl p write?)
    (let ((print (if write? write display)))
      (display #\()
      (let loop ((dl dl))
        (print (_dl-val dl))
        (let ((right (_dl-right dl)))
          (if right (begin (display " ")(loop right))
              (display #\)) )))))  )

(define (dlist a b c)
  (shared ((middle (_dl left   b right))
           (left   (_dl #false a middle))
           (right  (_dl middle c #false)))
   middle) )

(dlist 1 2 3)

(display (dlist 1 2 3))


Outputs:


(2 3)(2 3)(2 3)
(2 3)(2 3)


but when #:mutable is removed it fails in display not in shared. Further
I do not understand why my custom display code is called so often.
Lastly, I wonder if there is a benefit to specific orderings of the
shared clauses or not?

Thanks,

Marijn
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