[racket] Confusion with circular structs

You're right, that did it--and `shared' gives me what I expect with
s/set/seteq/ as well. Many thanks!

> (shared ([v0 (vertex 0 (seteq) succs0)]
           [v1 (vertex 1 preds1 (seteq))]
           [v2 (vertex 2 preds2 succs2)]
           [succs0 (seteq v1 v2)]
           [preds1 (seteq v0)]
           [preds2 (seteq v0 v2)]
           [succs2 (seteq v2)])
    (values v0 v1 v2))
#0=(vertex
    0
    (seteq)
    (seteq
     (vertex 1 (seteq #0#) (seteq))
     #2=(vertex 2 (seteq #0# #2#) (seteq #2#))))
#0=(vertex
    1
    (seteq
     #2=(vertex
         0
         (seteq)
         (seteq #0# #3=(vertex 2 (seteq #2# #3#) (seteq #3#)))))
    (seteq))
#0=(vertex
    2
    (seteq
     #1#
     #0#)
    (seteq #0#))


On Fri, Jul 26, 2013 at 3:49 PM, Ryan Culpepper <ryanc at ccs.neu.edu> wrote:

> It's probably because you're putting mutable data into an equal
> (hash-based) set, then mutating the data. That changes the keys, which
> causes problems for the underlying hash table.
>
> If you change each occurrence of 'set' to 'seteq' below, the program
> should run.
>
> Ryan
>
>
>
> On 07/26/2013 05:25 PM, Nicholas Labich wrote:
>
>> I'm running into what I find to be surprising behavior with circular
>> structures (vertices of a simple graph).
>>
>> Graphically (see code below to reproduce):
>> 0 --> 1 ;; no problem
>> 2-+      ;; also no problem
>>   ^-+
>>
>> But when I try to add an edge between 0 and 2, I hit an infinite loop at
>> line 88 of racket/set.rkt (in 5.3.3) or line 447 of
>> racket/private/set-types.rkt (in the latest 5.90.0.2), both of which are
>> the hash-proc for set's gen:equal+hash.
>>
>> Is this the expected behavior (after all, OCaml makes no guarantee of
>> termination for (=) on circular data)? I'm surprised that the self-edge
>> on 2 doesn't result in the same. I've also tried to reproduce the same
>> using `shared', but I'm assuming I still misunderstand its functionality
>> because the result is not quite what I expect it to be.
>>
>> Any insight would be much appreciated.
>>
>> Nick
>>
>> Welcome to Racket v5.90.0.2.
>>  > (struct vertex (value preds succs) #:mutable #:transparent)
>>  > (define v0 (vertex 0 (set) (set)))
>>  > (define v1 (vertex 1 (set) (set)))
>>  > (set-vertex-succs! v0 (set v1))
>>  > (set-vertex-preds! v1 (set v0)) ;; 0 --> 1
>>  > (equal? v0 (set-first (vertex-preds v1)))
>> #t
>>  > (define v2 (vertex 2 (set) (set)))
>>  > (set-vertex-preds! v2 (set v2)) ;; 2--+
>>  > (set-vertex-succs! v2 (set v2)) ;; ^--+
>>  > v2
>> #0=(vertex 2 (set #0#) (set #0#))
>> ;; try to add the already existing set
>>  > (set-vertex-succs! v0 (set-add (vertex-succs v0) v2)) ;; loops and
>> loops
>>    C-c C-c^Cuser break
>>    context...:
>>   .../racket/collects/racket/**private/set-types.rkt:447:3: hash-proc
>>   .../racket/collects/racket/**private/set-types.rkt:447:3: hash-proc
>>   .../racket/collects/racket/**private/set-types.rkt:447:3: hash-proc
>>   .../racket/collects/racket/**private/set-types.rkt:447:3: hash-proc...
>> ;; try to make a new set instead
>>  > (set-vertex-succs! v0 (set v1 v2))
>>    C-c C-c^Cuser break
>>    context...:
>>    .../racket/collects/racket/**private/set-types.rkt:447:3: hash-proc
>>    .../racket/collects/racket/**private/set-types.rkt:447:3: hash-proc
>>    .../racket/collects/racket/**private/set-types.rkt:447:3: hash-proc
>>    .../racket/collects/racket/**private/set-types.rkt:447:3: hash-proc...
>>  > (shared ([v0 (vertex 0 (set) succs0)]
>>             [v1 (vertex 1 preds1 (set))]
>>             [v2 (vertex 2 preds2 succs2)]
>>             [succs0 (set v1 v2)]
>>             [preds1 (set v0)]
>>             [preds2 (set v0 v2)]
>>             [succs2 (set v2)])
>>      (values v0 v1 v2))
>> (vertex 0 (set) (set #0=(vertex 2 (set #0#) (set #0#))))
>> (vertex 1 (set (vertex 0 (set) (set #0=(vertex 2 (set #0#) (set #0#)))))
>> (set))
>> #0=(vertex 2 (set #0#) (set #0#))
>> ;; I must be using `shared' incorrectly...
>>
>>
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>>
>
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