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Clojure! An Introduction to Zippers! ! Clojure User Group Paris – Nov 2013!

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#whoami   Jérémie Grodziski Software Designer and Programmer @jgrodziski [email protected] blog.zenmodeler.com   www.redsen.fr/blog  

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What’s the problem? I have a tree I want to modify ONE node and don’t want to rebuild the whole tree 3 4 7 2 4 8 5 9 3 6 => Zipper

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History

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Zipper fundamentals •  Purely functional data structure •  Think of it as a breacrumb or a zipper, that remember every direction you took from the root but starting from where you are, like an inverted glove

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Zipper functions

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Zipper creation

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Example means [] 0 [] 1A 1B 1C [] 2A 2B 2C [] ...etc

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Zipper navigation

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loc, node and root A loc (location) point to the current node along with the context (up, left and right) think: « like a pointer » to a particular node in the tree (node loc) returns the node at the loc (root loc) returns the root node (and not the loc!)

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Zipper edition

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Under the hood •  Zippers stores its “implementation” in metadata

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One last thing If you just want to walk a tree: use tree-seq then clojure.walk functions