Clojure

clojure.zip/seq-zip returns spurious nils during traversal

Details

  • Type: Defect Defect
  • Status: Open Open
  • Priority: Minor Minor
  • Resolution: Unresolved
  • Affects Version/s: None
  • Fix Version/s: None
  • Component/s: None
  • Labels:
  • Patch:
    Code

Description

Problem reported by Lee Spector on the mailing list:

https://groups.google.com/d/msg/clojure/8TL7IGmE7N0/u1xfgTOLDRgJ

Here's a quote from Lee's post describing the problem:

Here's an illustration, stepping through '(() 0) with next and printing the node at each step: 

(loop [z (zip/seq-zip '(() 0))] 
  (if (zip/end? z) 
    :done 
    (do (println (zip/node z)) 
      (recur (zip/next z))))) 

That produces: 

(() 0) 
() 
nil 
0 
:done 

I don't expect the nil to be there. 

The underlying cause is that seq-zip passes identity as the children argument to zipper. Applied to (), this returns (), which is truthy, leading zipper to descend into a non-existent subtree.

One natural solution would be to use seq in place of identity:

(defn seq-zip [root]
  (zipper seq?
          seq  ;; changed
          (fn [node children] (with-meta children (meta node)))
          root))

With this change, no nil is produced in the example above. Patch with this change forthcoming.

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  • Created:
    Updated: