Add examples.

docs/section-8-code-navigation-systems.md

@@ -5,15 +5,66 @@ permalink: code-navigation-systems
 
 # Code Navigation Systems
 
-Tree-sitter can be used in conjunction with its [tree query language](https://tree-sitter.github.io/tree-sitter/using-parsers#pattern-matching-with-queries) as a part of code navigation systems. An example of such a system can be seen in the `tree-sitter tag` command, which emits a textual dump of the interesting syntactic nodes in its file argument. This document exists to provide guidelines on the design and use of tree-sitter concepts to implement such systems.
+Tree-sitter can be used in conjunction with its [tree query language](https://tree-sitter.github.io/tree-sitter/using-parsers#pattern-matching-with-queries) as a part of code navigation systems. An example of such a system can be seen in the `tree-sitter tag` command, which emits a textual dump of the interesting syntactic nodes in its file argument. A notable application of this is GitHub's support for [search-based code navigation](https://docs.github.com/en/repositories/working-with-files/using-files/navigating-code-on-github#precise-and-search-based-navigation). This document exists to describe how to extend the
 
 ## Tagging and captures
 
 *Tagging* is the act of identifying the entities that can be named in a program. We use Tree-sitter queries to find those entities. Having found them, you use a syntax capture to label the entity and its name.
 
-The essence of a given tag lies in two pieces of data: the _kind_ of entity that is matched (usually a definition or a reference) and the _role_ of that entity, which describes how the entity is used (i.e. whether it's a class definition, function call, variable reference, and so on). Our convention is to use a syntax capture following the `@role.kind` capture name format, and another inner capture, always called `@name`, that pulls out the name of a given identifier.
+You can use the `tree-sitter tag` command to test out a given set of tags
 
-The below table describes a standard vocabulary for kinds and roles during the tagging process. User applications may extend (or only recognize a subset of) these capture names, but it is desirable to standardize on the names below when supported by a given system or language. Language communities that write tagging rules using these names can work out-of-the-box with a steadily increasing set of analysis tools
+The essence of a given tag lies in two pieces of data: the _kind_ of entity that is matched (usually a definition or a reference) and the _role_ of that entity, which describes how the entity is used (i.e. whether it's a class definition, function call, variable reference, and so on). Our convention is to use a syntax capture following the `@kind.role` capture name format, and another inner capture, always called `@name`, that pulls out the name of a given identifier.
+'
+You may optionally include a capture named `@doc `to bind a docstring. For convenience purposes, the tagging system provides two built-in functions, `#select-adjacent` and `#strip` that are convenient for removing comment syntax from a docstring. `#strip` takes a capture as its first argument and a regular expression, expressed as a quoted string. Any text patterns matched by the regular expression will be removed from the text associated with the passed capture. `#select-adjacent`, when passed two capture names, filters the text associated with the first capture so that only text adjacent to the second capture is preserved. This can be useful when writing queries that would otherwise include too much information in matched comments.
+
+## Examples
+
+An [example query](https://github.com/tree-sitter/tree-sitter-python/blob/78c4e9b6b2f08e1be23b541ffced47b15e2972ad/queries/tags.scm#L4-L5) follows, one that recognizes Python function definitions and captures their declared name. The `function_definition` syntax node is defined in the [Python Tree-sitter grammar](https://github.com/tree-sitter/tree-sitter-python/blob/78c4e9b6b2f08e1be23b541ffced47b15e2972ad/grammar.js#L354).
+
+``` scheme
+(function_definition
+  name: (identifier) @name) @definition.function
+```
+
+A more sophisticated query can be found in the [JavaScript Tree-sitter repository](https://github.com/tree-sitter/tree-sitter-javascript/blob/fdeb68ac8d2bd5a78b943528bb68ceda3aade2eb/queries/tags.scm#L63-L70):
+
+``` scheme
+(assignment_expression
+  left: [
+    (identifier) @name
+    (member_expression
+      property: (property_identifier) @name)
+  ]
+  right: [(arrow_function) (function)]
+) @definition.function
+```
+
+An even more sophisticated query is in the [Ruby Tree-sitter repository](https://github.com/tree-sitter/tree-sitter-ruby/blob/1ebfdb288842dae5a9233e2509a135949023dd82/queries/tags.scm#L24-L43), which uses built-in functions to strip the Ruby comment character (`#`) from the docstrings associated with a class or singleton-class declaration, then selects only the docstrings adjacent to the node matched as `@definition.class`.
+
+``` scheme
+(
+  (comment)* @doc
+  .
+  [
+    (class
+      name: [
+        (constant) @name
+        (scope_resolution
+          name: (_) @name)
+      ]) @definition.class
+    (singleton_class
+      value: [
+        (constant) @name
+        (scope_resolution
+          name: (_) @name)
+      ]) @definition.class
+  ]
+  (#strip! @doc "^#\\s*")
+  (#select-adjacent! @doc @definition.class)
+)
+```
+
+The below table describes a standard vocabulary for kinds and roles during the tagging process. User applications may extend (or only recognize a subset of) these capture names, but it is desirable to standardize on the names below when supported by a given system or language. Language communities that write tagging rules using these names can work out-of-the-box with a steadily increasing set of analysis tools.
 
 | Category                 | Tag                         |
 |--------------------------|-----------------------------|
@@ -26,8 +77,4 @@ The below table describes a standard vocabulary for kinds and roles during the t
 | Class reference          | `@reference.class`          |
 | Interface implementation | `@reference.implementation` |
 
-To communicate the associated identifier inside one of these syntactic classes, capture the identifier within as `@name`.
-
-## `tree-sitter graph`
-
-Documentation forthcoming.
+By convention, tags for a given language are made available in a `queries/tags.scm `file in that language's repository.