This package provides classes representing XPath values (that is, the results of an expression). There are different classes for different types of value.

The principal class is:

Value:
This represents the result of evaluating an expression. But a Value is also an expression in its own right, reflecting the fact that literal values can be used syntactically wherever expressions can be used. `AtomicValue` is a subclass of `Value` that represents an atomic value: this in turn has subclasses for the different built-in data types: StringValue, NumericValue, BooleanValue, DateValue, and so on.

There are two classes used to represent integer values: `IntegerValue` for integers that fit comfortably in 64 bits, and `BigIntegerValue` for anything longer. Built-in subtypes of xs:integer, such as `xs:int` and `xs:short`, are always represented using an `IntegerValue`, except for `xs:unsignedLong`, which uses a `BigIntegerValue`.

In general a value is a sequence. A sequence that is instantiated in memory is represented by a `SequenceExtent` object. The code tries to pipeline execution so that a `SequenceExtent` is created as rarely as possible.

When an expression is evaluated lazily, the result is a `Closure`. A Closure contains the original expression, and a copy of the run-time context at the time evaluation was requested: that is, all the local variables and other information that it depends on, such as the context item. There are two kinds of Closure, and the system decides at compile time which to use. An ordinary Closure re-evaluates the expression every time the value is referenced; this is used only when the compiler decides that the value is likely to be referenced only once. A MemoClosure remembers the value the first time it is needed, so that subsequent references do not cause repeated evaluation of the expression.

Michael H. Kay
Saxonica Limited
9 February 2005