docs: standard functions

docs/docs/guides/jsonpath-functions.md

@@ -1,11 +1,75 @@
 # JSONPath Functions
 
-TODO:
+## Standard functions
 
-## `match()`
+These are the standard, built-in function available to JSONPath [filters](./jsonpath-syntax.md#filters). You can also create your own [function extensions](#function-extensions).
 
-TODO:
+### `count()`
+
+```typescript
+count(nodes: JSONPathNodeList): number
+```
+
+Return the number of nodes in a node list. Usually, `count()` will be given a [filter query](./jsonpath-syntax.md#filter-queries) as its argument, and a call to `count()` must be part of a comparison expression.
+
+```text title="Example query"
+$.users[?count(@.*) > 2]
+```
+
+### `length()`
+
+```typescript
+length(value: JSONValue): number | undefined
+```
+
+Return the length of a string or array, or the number of items in an object. Usually, `length()` will be given a [filter query](./jsonpath-syntax.md#filter-queries) as its argument, and a call to `length()` must be part of a comparison expression.
+
+```text title="Example query"
+$.users[?length(@) > 2]
+```
+
+### `match()`
+
+```typescript
+match(value: string, pattern: string): boolean
+```
+
+Return `true` if _value_ is a full match to the regular expression _pattern_, or `false` otherwise.
+
+```text title="Example query"
+$.users[?match(@.name, '[Ss].*')]
+```
+
+### `search()`
+
+```typescript
+search(value: string, pattern: string): boolean
+```
+
+Return `true` if _value_ contains _pattern_, or `false` otherwise.
+
+```text title="Example query"
+$.users[?search(@.name, '[Aa]')]
+```
+
+### `value()`
+
+```typescript
+value(nodes: JSONPathNodeList): JSONValue | undefined
+```
+
+Return the value associated with the first node in _nodes_, if _nodes_ has exactly one [`JSONPathNode`](../api/classes/jsonpath.JSONPathNode.md). Usually, `value()` will be called with a [filter query](./jsonpath-syntax.md#filter-queries) as its argument.
+
+:::info
+[Filter queries](./jsonpath-syntax.md#filter-queries) that can result in at most one node are known as "singular queries", and all singular queries will be implicitly replaced with their value as required, without the use of `value()`. `value()` is useful when you need the value from a query that can, theoretically, return multiple nodes.
+:::
+
+## Well-typedness
+
+The JSONPath specification defines a [type system for function expressions](https://datatracker.ietf.org/doc/html/draft-ietf-jsonpath-base-20#name-type-system-for-function-ex), and rules for how those types can be used within an expression. JSON P3 will throw a [JSONPathTypeError](../api/classes/jsonpath.JSONPathTypeError.md) as query compile time if it contains expressions that are not deemed to be well-typed.
+
+Please see [Section 2.4.3](https://datatracker.ietf.org/doc/html/draft-ietf-jsonpath-base-20#name-well-typedness-of-function-) _Well-Typedness of Function Expressions_.
 
-## `search()`
+## Function extensions
 
 TODO:

docs/docs/guides/jsonpath-syntax.md

@@ -141,6 +141,19 @@ Selectively include child nodes from the current selection using filters of the
 - an existence test on the result of a JSONPath query,
 - or the truthiness of some function calls, depending on the function's return type.
 
+```text
+$.users[[email protected]]
+```
+
+```json title="Example output"
+[
+  { "name": "John", "score": 86, "admin": true },
+  { "name": "Sally", "score": 84, "admin": false }
+]
+```
+
+### Filter queries
+
 Inside a filter expression, `@` is the _current node identifier_, starting a new JSONPath query with the current node at the root.
 
 ```text
@@ -177,7 +190,7 @@ $.users[[email protected] > 85 && @.score < 100].name
 ["John"]
 ```
 
-### Functions
+### Filter functions
 
 Filter expressions can include calls to predefined functions. For example, the [`match()`](./jsonpath-functions.md#match) function matches nodes against a regular expression, if that node is a string value.
 
@@ -215,7 +228,7 @@ $.users[0, [email protected] == 'Sally', 'foo'].score
 
 ### Recursive descent
 
-The recursive descent segment (`..[<selectors>]`) selects all nodes, recursively, beneath the current selection in the JSON document tree. There mst be at least one selector. In this example we use shorthand notation for a property name (`score`), but a bracketed selector list is OK too.
+The recursive descent segment (`..[<selectors>]`) visits all nodes, recursively, beneath the current selection in the JSON document tree. There mst be at least one selector. In this example we use shorthand notation for a property name (`score`), but a bracketed selector list is OK too.
 
 ```text
 $..score

docs/docs/intro.mdx

@@ -9,13 +9,15 @@ import TabItem from "@theme/TabItem";
 
 JSON P3 implements JSONPath following the [IETF JSONPath draft](https://datatracker.ietf.org/doc/html/draft-ietf-jsonpath-base-20) specification, JSON Pointer described by [RFC 6901](https://datatracker.ietf.org/doc/html/rfc6901), and JSON Patch described by [RFC 6902](https://datatracker.ietf.org/doc/html/rfc6902).
 
-**JSONPath** is a mini language for selecting values from JSON-like data. A JSONPath query has the potential to return multiple values from a data structure, along with their locations. This implementation is non-evaluating and read-only. We strictly follow the [IETF JSONPath draft](https://datatracker.ietf.org/doc/html/draft-ietf-jsonpath-base-20).
+**[JSONPath](./quick-start.md#jsonpath)** is a mini language for selecting values from JSON-like data. A JSONPath query has the potential to return multiple values from a data structure, along with their locations. This implementation is non-evaluating and read-only. We strictly follow the [IETF JSONPath draft](https://datatracker.ietf.org/doc/html/draft-ietf-jsonpath-base-20).
 
-**JSON Pointer** is a syntax for targeting a single value in JSON-like data. JSON Pointers can be _resolved_ against data to retrieve the value, or used as part of a JSON Patch operation.
+**[JSON Pointer](./quick-start.md#json-pointer)** is a syntax for targeting a single value in JSON-like data. JSON Pointers can be _resolved_ against data to retrieve the value, or used as part of a JSON Patch operation.
 
-**JSON Patch** is a standard for describing update operations to perform on JSON-like data. You can _apply_ a patch to _add_, _remove_, _replace_, _copy_ and _move_ values within a JSON document.
+**[JSON Patch](./quick-start.md#json-patch)** is a standard for describing update operations to perform on JSON-like data. You can _apply_ a patch to _add_, _remove_, _replace_, _copy_ and _move_ values within a JSON document.
 
+:::info
 We use the term _JSON-like data_ to describe arbitrary, possibly nested, JavaScript arrays and objects, plus primitive strings, numbers, booleans and `null`, as you might expect from `JSON.parse()`. When traversing JSON-like data, we only resolve an object's direct properties, and the `length` property of arrays and strings is ignored.
+:::
 
 ## Install