This package (introduced in Saxon 9.9) provides methods to manipulate XDM values using Java 8 streams.

A value in the XDM data model is represented by an instance of {@link net.sf.saxon.s9api.XdmValue}. This is in general a sequence; the members of the sequence are instances of {@link net.sf.saxon.s9api.XdmItem}. {@code XdmItem} is an abstract class; its concrete subclasses include {@link net.sf.saxon.s9api.XdmNode} for nodes, {@link net.sf.saxon.s9api.XdmAtomicValue} for atomic values, {@link net.sf.saxon.s9api.XdmFunctionItem} for function items, {@link net.sf.saxon.s9api.XdmMap} for maps, and {@link net.sf.saxon.s9api.XdmArray} for arrays.

Given an `XdmNode` `N`, it is possible to select other nodes using an expression such as `N.select(child("author")).asNode()`. The way this works is as follows:

- The `select()` method (which applies to any `XdmValue`, not only an `XdmNode`) takes as its argument a {@link net.sf.saxon.s9api.streams.Step}, and returns as its result an {@link net.sf.saxon.s9api.streams.XdmStream}. A `Step` is a function that takes an `XdmItem` as input, and returns a stream of items as its result; the `select()` method combines these streams into a single stream (in much the same way as the Java 8 `flatMap()` operation) and returns the result.
- `child("author")` invokes a static method in class {@link net.sf.saxon.s9api.streams.Steps}, which delivers a `Step` whose effect is to find the children of a supplied node that have local name `"author"`, returning these as a Stream. The `Steps` class provides a large collection of useful implementations of `Step`, including support for all the XPath axes (parent, child, descendant, following-sibling and so on).
- The class `XdmStream` (which will typically not be used explicitly) implements the standard Java 8 {@link java.util.streams.Stream} class; it does so by wrapping a standard Java 8 stream and delegating all standard methods to the wrapped stream. By subclassing the standard `Stream` interface, however, it is able to supply additional methods appropriate to streams of XDM items. For example, the `asNode()` example terminates the Stream pipeline by converting the result of the stream to a single `XdmNode` value (an unchecked exception occurs if the content of the stream is anything other than a single node).

The power of the approach rests in the range of `Step` implementations available, and the way these can be combined; and in the terminal operations to deliver the result of a stream in a useful way. Although many standard implementations are available, these can be augmented with user-written methods; since a {@code Step} is just a Java 8 {@code Function}, and an {@code XdmStream} is just a Java 8 {@code Stream}, all the standard Java 8 facilities for creating and combining functions and streams are available.

## Steps

The steps that are available "off the shelf" from the {@link net.sf.saxon.s9api.streams.Steps} class include the following:

- For each of the 13 XPath axes, taking `child` as an example, four `Step` implementations are provided:

  - `child()` selects all the children of a node.
  - `child("lname")` selects all the children of a node that have the local name "lname".
  - `child("ns", "lname")` selects all the children of a node that have the namespace URI "ns" and local name "lname".
  - `child(predicate)` selects all the children of a node that satisfy the given predicate. The predicate may be any Java 8 {@link java.util.functions.Predicate}, but the class {@link net.sf.saxon.s9api.streams.Predicates} provides some off-the-shelf predicates that are particularly designed for navigating XDM information.

- Two steps `S` and `T` may be combined into a single step using the `then` method: for example `child("X").then(attribute("A"))`.

- More generally, any sequence of steps may be combined using the `path()` function, for example `path(child("table"), child("thead"), child("trow"), attribute("class"))`

- Where the steps in a path are the most commonly used variety, a path may be written using an abbreviated notation: `path("table", "thead", "trow", "@class")` is equivalent to the previous example.

- The results of two steps may also be concatenated: for example `child("author").cat(child("editor"))` concatenates the results of the two steps into a single stream.

- The `atomize()` step reduces nodes (and arrays) to atomic values, as defined in the XPath specification.

- If `S` is a `Step`, and `P` is a `Predicate`, then `S.where(P)` is also a `Step`, which selects those elements selected by `S` that satisfy `P`. For example `child().where(isElement())` selects those children of the starting node that are elements. (But this particular example can be written more easily as `child(isElement())`).

## Predicates

Predicates can be used either in the `where()` method to construct a new `Step`, or they may be used in the standard Java 8 `filter()` method to filter the items in a stream. Any predicate may be used in either context. The utility class {@link net.sf.saxon.s9api.streams.Predicates} provides a range of predicates that are particularly suited to XDM navigation.

These predicates include:

- Predicates `isNode()`, `isAtomicValue()`, `isMap()`, `isArray()` etc to test whether an `XdmItem` is a particular kind of item;
- Predicates `isElement()`, `isAttribute()`, `isText()` etc to test the kind of a node;
- The predicate `hasType()` to test whether an item matches a specific item type: for example `hasType(ItemType.XS_DATE_TIME)` tests whether it is an instance of `xs:dateTime`;
- The predicate `eq("string")` tests whether the string value of the item is equal to "string"
- The predicate `eq(XdmAtomicValue)` performs a typed comparison, for example comparing two values as numbers
- The predicate `matchesRegex("regex")` tests whether the string value of the item matches the regular expression "regex"
- If `S` is a `Step` and `P` is a `Predicate`, then `some(S, P)` is a predicate that returns true if some item returned by `S` satisfies `P`; similarly `every(S, P)` tests if all items returned by `S` satisfy `P`. For example `some(attribute("id"), eq("A123"))` is true for an element that has an `id` attribute equal to "A123". This particular condition can also be expressed more concisely as `eq(attribute("id"), "A123")`

## Operations on XDM streams

An `XdmStream` (as delivered by `XdmValue.select(step)`) is an implementation of a Java 8 Stream, so all the standard methods on `Stream` are available: for example `filter`, `map()`, `flatMap`, `reduce`, `collect`. Where appropriate, these are specialized to return an `XdmStream` rather than a generic `Stream`.

`XdmStream` provides some additional terminal operations designed to make it convenient to convert the contents of the stream into usable form. These include:

- `first()` - deliver the first item of the stream
- `last()` - deliver the last item of the stream
- `at()` - deliver the item at position N the stream
- `exists()` - return true if the stream is non-empty
- `subStream()` - deliver a stream containing a subsequence of the input stream
- `asXdmValue()` - deliver the contents as an `XdmValue`
- `asList()` - deliver the contents as a `List<XdmItem>`
- `asListOfNodes()` - deliver the contents as a `List<XdmNode>`
- `asOptionalNode()` - deliver the contents as an `Optional<XdmNode>`
- `asNode()` - deliver the contents as a single `XdmNode`
- `asListOfAtomic()` - deliver the contents as a `List<XdmNode>`
- `asOptionalAtomic()` - deliver the contents as an `Optional<XdmNode>`
- `asAtomic()` - deliver the contents as a single `XdmNode`
- `asOptionalString()` - deliver the contents as an `Optional<String>` by taking the string value of each item
- `asString()` - deliver the contents as a single `String`

The choice of terminal operation determines the return type (for example `asOptionalNode()` returns `Optional<XdmNode>)`, and also causes a run-time check that the value actually conforms to these expectations. For example, if `asNode()` is used, then an unchecked exception occurs if the sequence has a length other than 1 (one), or if its single item is not a node.

Other ways of generating an `XdmStream` include (TODO!!!):

- As the result of an XPath expression, using the method `XPathSelector.stream()`
- As the result of an XQuery expression, using the method `XQueryEvaluator.stream()`
- As a `StreamDestination` capturing the result of an XSLT transformation