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  1. ; sets without replacement
  2.  
  3. (define s (list)) ; set of current items
  4. (define t (list)) ; tombstones of deleted items
  5.  
  6. (define (add s x) ; add x to set s
  7.   (if (member x s)
  8.       (error 'add "can't add already existent element")
  9.       (if (member x t)
  10.           (error 'add "can't add previously existent element")
  11.           (begin (set! s (cons x s)) s))))
  12.  
  13. (define (del s x) ; remove x from set s
  14.   (if (not (member x s))
  15.       (error 'del "can't delete non-existent element")
  16.       (begin (set! s (remove x s))
  17.              (set! t (cons x t))
  18.              s)))
  19.  
  20. (set! s (add s 1))
  21. (set! s (add s 3))
  22. (set! s (add s 5))
  23.  
  24. (set! s (add s 3))
  25. (set! s (del s 3))
  26. (set! s (add s 3))
Runtime error #stdin #stdout #stderr 0.01s 7636KB
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Standard input is empty
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Standard output is empty
stderr
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Error: (add) can't add already existent element

	Call history:

	<eval>	  [add] (member x s)
	<eval>	  [add] (member x t)
	<eval>	  [add] (cons x s)
	<syntax>	  (set! s (add s 5))
	<syntax>	  (##core#set! s (add s 5))
	<syntax>	  (add s 5)
	<eval>	  (add s 5)
	<eval>	  [add] (member x s)
	<eval>	  [add] (member x t)
	<eval>	  [add] (cons x s)
	<syntax>	  (set! s (add s 3))
	<syntax>	  (##core#set! s (add s 3))
	<syntax>	  (add s 3)
	<eval>	  (add s 3)
	<eval>	  [add] (member x s)
	<eval>	  [add] (error (quote add) "can't add already existent element")	<--
https://ideone.com/0wwfjN
language:
Scheme (chicken 4.13)
created:
5 years ago
visibility:
secret

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