lazy recursive definition giving incorrect results

Question

If you define a global data var in terms of a lazy sequence referring to that same var, you can get different results depending on the chunkiness of laziness of the functions being used to build the collection.

Clojure's lazy sequences don't promise to support this, but they shouldn't return wrong answers. In the example given in http://groups.google.com/group/clojure/browse_thread/thread/1c342fad8461602d (and repeated below), Clojure should not return bad data. An error message would be good, and even an infinite loop would be more reasonable than the current behavior.

(Similar issue reported here: https://groups.google.com/d/topic/clojure/yD941fIxhyE/discussion)

(def nums (drop 2 (range Long/MAX_VALUE)))
(def primes (cons (first nums)
             (lazy-seq (->>
               (rest nums)
               (remove
                 (fn [x]
                   (let [dividors (take-while #(<= (* % %) x)
primes)]
                     (println (str "primes = " primes))
                     (some #(= 0 (rem x %)) dividors))))))))
(take 5 primes)

It prints out:
(primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
2 3 5 7 9)

Answer 1

Comment made by: importer

Converted from http://www.assembla.com/spaces/clojure/tickets/457

Answer 2

Comment made by: aaron

Stu and Rich talked about making this an error, but it would break some existing code to do so.

Answer 3

Comment made by: richhickey

Is there a specific question on this?

Answer 4

Comment made by: aaron

Stu, you and I went over this but I can't remember exactly what the question was here.

Answer 5

Comment made by: cgrand

Tentative patch attached.
Have you an example of existing code which is broken by such a patch (as mention by Aaron Bedra)?

Answer 6

Comment made by: richhickey

The patch intends to do what? We have only a problem description and code. Please enumerate the plan rather than make us decipher the patch.

As a first principle, I don't want Clojure to promise that such recursively defined values are possible.

Answer 7

Comment made by: cgrand

The proposal here is to catch recursive seq realization (ie when computing the body of a lazy-seq attempts to access the same seq) and throw an exception.

Currently when such a case happens, the recursive access to the seq returns nil. This results in incorrect code seemingly working but producing incorrect results or even incorrect code producing correct results out of luck (see https://groups.google.com/d/topic/clojure/yD941fIxhyE/discussion for such an example).

So this patch moves around the modification to the LazySeq state (f, sv and s fields) before all potentially recursive method call (.sval in the while of .seq and .invoke in .sval) so that, upon reentrance, the state of the LazySeq is coherent and able to convey the fact the seq is already being computed.

Currently a recursive call may find f and sv cleared and concludes the computation is done and the result is in s despite s being unaffected yet.

Currently:
|State|f|sv|s|
| :-- | :-- | :-- | :-- | :-- |
|Unrealized|not null|null|null|
|Realized|null|null|anything|
|Being realized/recursive call from fn.invoke|not null|null|null|
|Being realized/recursive call from ls.sval|null|null|null|
Note that "Being realized" states overlap with Unrealized or Realized.
(NB: "anything" includes null)

With the patch:
|State|f|sv|s|
| :-- | :-- | :-- | :-- | :-- |
|Unrealized|not null|null|null|
|Realized|null|null|anything but this|
|Being realized|null|null|this|

Answer 8

Comment made by: jafingerhut

That last comment, Christophe, goes a long way to explaining the idea to me, at least. Any chance comments with similar content could be added as part of the patch?

Answer 9

Comment made by: cgrand

New patch with a comment explaining the expected states.
Note: I tidied the states table up.

// Before calling user code (f.invoke() in sval and, indirectly, // ((LazySeq)ls).sval() in seq -- and even RT.seq() in seq), ensure that // the LazySeq state is in one of these states: // // State f sv // ================================ // Unrealized not null null // Realized null null // Being realized null this

CLJ-1119 is also fixed by this patch.

Answer 10

Comment made by: jafingerhut

Patch clj-457-3.diff is identical to Christophe's CLJ-457-2.diff, except it has been updated to no longer conflict with the commit made for CLJ-949 on Nov 22, 2013, which basically removed the try/catch in method sval(). It passes tests, and I don't see anything wrong with it, but it would be good if Christophe could give it a look, too.

Answer 11

Comment made by: alexmiller

No longer reproducible

Answer 12

_Comment made by: bronsa_

Alex, the posted example is no longer reproducible because `(range)` does not produce a chunked-seq anymore.
OTOH `(range n)` does so replacing `(range)` with something like `(range Long/MAX_VALUE)` in the code example in the ticket description is sufficient to resurface the bug:




Clojure 1.8.0-master-SNAPSHOT
user=> (def nums (drop 2 (range Long/MAX_VALUE)))
#'user/nums
(def primes (cons (first nums)
             (lazy-seq (->>
               (rest nums)
               (remove
                 (fn [x]
                   (let [dividors (take-while #(<= (* % %) x)
primes)]
                     (println (str "primes = " primes))
                     (some #(= 0 (rem x %)) dividors))))))))
#'user/primes
user=> (take 5 primes)
(primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
primes = (2)
2 3 5 7 9)

Answer 13

Reference: https://clojure.atlassian.net/browse/CLJ-457 (reported by alex+import)