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# Data Structure Utilities
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Comprehensive utilities for traversing, transforming, and navigating complex data structures through specialized namespaces: clojure.walk for tree traversal, clojure.datafy for data transformation, and clojure.zip for functional tree editing.
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## Capabilities
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### Tree Walking (clojure.walk)
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Generic tree traversal functions for recursively processing nested data structures.
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```clojure { .api }
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(clojure.walk/walk inner outer form)
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;; Core traversal function - applies inner to each element, then outer to result
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;; inner: function applied to each sub-element
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;; outer: function applied to reconstructed structure
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;; form: data structure to traverse
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;; Returns: transformed data structure
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(clojure.walk/prewalk f form)
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;; Pre-order traversal - calls f on each sub-form before recursing
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;; f: transformation function
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;; form: data structure to traverse
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;; Returns: transformed data structure
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(clojure.walk/postwalk f form)
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;; Post-order traversal - calls f on each sub-form after recursing
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;; f: transformation function  
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;; form: data structure to traverse
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;; Returns: transformed data structure
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(clojure.walk/prewalk-demo form)
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;; Demonstrates prewalk behavior by printing each walked form
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;; form: data structure to walk and print
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;; Returns: original form (unchanged)
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(clojure.walk/postwalk-demo form)
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;; Demonstrates postwalk behavior by printing each walked form
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;; form: data structure to walk and print
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;; Returns: original form (unchanged)
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```
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### Tree Replacement (clojure.walk)
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Higher-level transformation functions built on tree walking.
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```clojure { .api }
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(clojure.walk/prewalk-replace smap form)
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;; Pre-order replacement using substitution map
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;; smap: map of {old-value new-value} replacements
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;; form: data structure to transform
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;; Returns: transformed data structure
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(clojure.walk/postwalk-replace smap form)
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;; Post-order replacement using substitution map
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;; smap: map of {old-value new-value} replacements
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;; form: data structure to transform
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;; Returns: transformed data structure
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(clojure.walk/keywordize-keys m)
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;; Recursively transforms all map keys from strings to keywords
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;; m: nested data structure containing maps
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;; Returns: transformed structure with keyword keys
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(clojure.walk/stringify-keys m)
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;; Recursively transforms all map keys from keywords to strings
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;; m: nested data structure containing maps
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;; Returns: transformed structure with string keys
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(clojure.walk/macroexpand-all form)
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;; Recursively performs all possible macroexpansions in form
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;; form: Clojure form potentially containing macros
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;; Returns: fully macroexpanded form
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```
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### Data Transformation (clojure.datafy)
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Protocol-based data transformation for converting objects to data representations.
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```clojure { .api }
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(clojure.datafy/datafy x)
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;; Attempts to return x as data using clojure.core.protocols/datafy
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;; x: object to transform to data representation
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;; Returns: data representation of x with metadata about original
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(clojure.datafy/nav coll k v)
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;; Returns possibly transformed v in context of coll and key k
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;; coll: containing collection
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;; k: key/index context (or nil)
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;; v: value to potentially transform
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;; Returns: transformed value via clojure.core.protocols/nav
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```
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### Functional Zippers (clojure.zip)
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Functional tree editing with location-aware navigation and modification.
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#### Zipper Creation
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```clojure { .api }
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(clojure.zip/zipper branch? children make-node root)
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;; Creates a new zipper structure for arbitrary trees
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;; branch?: predicate fn - true if node can have children
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;; children: fn returning seq of node's children
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;; make-node: fn creating new branch node from existing node and children
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;; root: root node of tree
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;; Returns: zipper positioned at root
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(clojure.zip/seq-zip root)
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;; Returns zipper for nested sequences
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;; root: root sequence
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;; Returns: sequence zipper
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(clojure.zip/vector-zip root)
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;; Returns zipper for nested vectors
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;; root: root vector
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;; Returns: vector zipper
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(clojure.zip/xml-zip root)
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;; Returns zipper for XML elements (as from xml/parse)
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;; root: root XML element
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;; Returns: XML zipper
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```
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#### Zipper Navigation
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```clojure { .api }
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(clojure.zip/node loc)
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;; Returns the node at current location
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;; loc: zipper location
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;; Returns: node at location
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(clojure.zip/branch? loc)
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;; Returns true if node at location is a branch
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;; loc: zipper location
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;; Returns: boolean
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(clojure.zip/children loc)
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;; Returns seq of children of branch node at location
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;; loc: zipper location (must be branch)
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;; Returns: seq of child nodes
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(clojure.zip/down loc)
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;; Returns location of first child, nil if no children
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;; loc: zipper location
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;; Returns: child location or nil
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(clojure.zip/up loc)
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;; Returns location of parent, nil if at top
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;; loc: zipper location
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;; Returns: parent location or nil
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(clojure.zip/left loc)
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;; Returns location of left sibling, nil if none
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;; loc: zipper location
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;; Returns: left sibling location or nil
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(clojure.zip/right loc)
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;; Returns location of right sibling, nil if none
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;; loc: zipper location
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;; Returns: right sibling location or nil
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(clojure.zip/leftmost loc)
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;; Returns location of leftmost sibling
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;; loc: zipper location
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;; Returns: leftmost sibling location
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(clojure.zip/rightmost loc)
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;; Returns location of rightmost sibling
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;; loc: zipper location
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;; Returns: rightmost sibling location
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(clojure.zip/next loc)
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;; Returns next location in depth-first walk, nil if end
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;; loc: zipper location
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;; Returns: next location or nil
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(clojure.zip/prev loc)
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;; Returns previous location in depth-first walk, nil if beginning
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;; loc: zipper location
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;; Returns: previous location or nil
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```
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#### Zipper Query Functions
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```clojure { .api }
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(clojure.zip/path loc)
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;; Returns seq of nodes leading to this location
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;; loc: zipper location
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;; Returns: seq of ancestor nodes
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(clojure.zip/lefts loc)
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;; Returns seq of left siblings of this location
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;; loc: zipper location
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;; Returns: seq of left sibling nodes
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(clojure.zip/rights loc)
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;; Returns seq of right siblings of this location
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;; loc: zipper location
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;; Returns: seq of right sibling nodes
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(clojure.zip/root loc)
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;; Returns root node of entire tree
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;; loc: zipper location
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;; Returns: root node
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(clojure.zip/end? loc)
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;; Returns true if location represents end of depth-first walk
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;; loc: zipper location
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;; Returns: boolean
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```
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#### Zipper Editing
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```clojure { .api }
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(clojure.zip/replace loc node)
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;; Returns new location with node replaced
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;; loc: zipper location
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;; node: replacement node
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;; Returns: new location with replaced node
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(clojure.zip/edit loc f & args)
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;; Returns location with node replaced by (apply f node args)
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;; loc: zipper location
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;; f: transformation function
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;; args: additional arguments to f
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;; Returns: new location with edited node
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(clojure.zip/insert-left loc item)
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;; Returns location with item inserted as left sibling
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;; loc: zipper location
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;; item: node to insert
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;; Returns: new location
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(clojure.zip/insert-right loc item)
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;; Returns location with item inserted as right sibling
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;; loc: zipper location
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;; item: node to insert
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;; Returns: new location
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(clojure.zip/insert-child loc item)
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;; Returns location with item inserted as leftmost child
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;; loc: zipper location (must be branch)
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;; item: node to insert
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;; Returns: new location
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(clojure.zip/append-child loc item)
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;; Returns location with item inserted as rightmost child
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;; loc: zipper location (must be branch)
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;; item: node to insert
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;; Returns: new location
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(clojure.zip/remove loc)
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;; Returns location with node removed
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;; loc: zipper location
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;; Returns: new location with node removed
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```
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**Usage Examples:**
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```clojure
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;; Tree walking examples
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(require '[clojure.walk :as walk])
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;; Transform all numbers in nested structure
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(walk/postwalk 
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  #(if (number? %) (* % 2) %)
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  {:a 1 :b [2 3 {:c 4}]})
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;; => {:a 2, :b [4 6 {:c 8}]}
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;; String keys to keyword keys
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(walk/keywordize-keys {"a" 1 "b" {"c" 2}})
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;; => {:a 1, :b {:c 2}}
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;; Replace specific values
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(walk/postwalk-replace {'old 'new :a :replaced}
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                       '(old data (:a value) old))
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;; => (new data (:replaced value) new)
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;; Functional zipper examples  
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(require '[clojure.zip :as zip])
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;; Navigate nested vector structure
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(def data [[1 2] [3 [4 5]] 6])
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(def z (zip/vector-zip data))
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;; Navigate to deeply nested value
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(-> z
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    zip/down          ; into first level: [1 2]
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    zip/right         ; to [3 [4 5]]
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    zip/down          ; into [3 [4 5]]
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    zip/right         ; to [4 5]
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    zip/down          ; into [4 5] 
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    zip/right         ; to 5
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    zip/node)         ; => 5
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;; Edit nested structure
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(-> z
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    zip/down          ; to [1 2]
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    zip/right         ; to [3 [4 5]]
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    zip/down          ; into [3 [4 5]]
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    (zip/edit inc)    ; change 3 to 4
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    zip/root)         ; => [[1 2] [4 [4 5]] 6]
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;; Add new elements
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(-> z
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    zip/down                    ; to [1 2]
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    (zip/insert-right [7 8])   ; insert new vector
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    zip/root)                   ; => [[1 2] [7 8] [3 [4 5]] 6]
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;; Tree traversal with zippers
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(defn find-in-tree [pred z]
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  (loop [loc z]
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    (cond
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      (zip/end? loc) nil
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      (pred (zip/node loc)) loc
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      :else (recur (zip/next loc)))))
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;; Find first even number
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(let [z (zip/vector-zip [1 [2 3] [4 [5 6]]])]
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  (when-let [loc (find-in-tree even? z)]
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    (zip/node loc)))  ; => 2
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;; Data transformation with datafy
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(require '[clojure.datafy :as d])
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;; Transform exception to data
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(try
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  (/ 1 0)
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  (catch Exception e
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    (d/datafy e)))
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;; => returns map with exception details
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;; Working with refs and atoms
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(let [a (atom {:count 42})]
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  (d/datafy a))  ; => [{\count 42}] with metadata
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```