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

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:

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:

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

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Alex Miller February 10, 2016 at 3:38 PM

No worries, just wanted to know if something was still pending - I will wait to prescreen it.

Michał Marczyk February 10, 2016 at 3:09 PM

Hey Alex, I was going to attach a separate patch with a proposal for a docstring adjustment along the lines suggested above (will do that tonight). No change to the code, though, and I guess not worth assigning the ticket – sorry about the unnecessary ping.

Alex Miller February 8, 2016 at 10:36 PM

Michał, can I ask why you assigned this to yourself - was there something you planned to add?

Michał Marczyk December 31, 2013 at 11:52 PM

Note that the docstring of clojure.zip/zipper asks that the children argument return a seq of children. The rest of clojure.zip, however, expects nil to be returned when there are no children, as evidenced by this problem.

One could argue that this behaviour of the rest of clojure.zip should be fixed, but I think it makes sense and is convenient. Perhaps the docstring should be adjusted, though.

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Created December 31, 2013 at 11:42 PM
Updated February 10, 2016 at 3:38 PM