XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (261 page)

The scope of a global variable is the entire stylesheet, including any stylesheets that are included or imported. A global variable may even be referenced before it is declared. The only constraint is that circular definitions are not allowed; if variable
x
is defined in terms of
y
, then
y
may not be defined directly or indirectly in terms of
x
.

The scope of a local variable is block-structured; it may be referenced in any following sibling element or in a descendant of a following sibling. This is illustrated in
Figure 6-16
.

Figure 6-16

The diagram shows, for a variable X, the elements that may contain a reference to X; the shaded elements may refer to X, and the unshaded elements may not. Specifically, a local variable may be referenced in any following sibling element, or in a descendant of a following sibling. It cannot be referenced within its own descendants, and it goes out of scope when the end tag for its parent element is encountered. Unlike global variables, a forward reference to a local variable is not allowed. If you think of XSLT instructions as being like the statements in a block-structured language such as C, Java, or JavaScript, and the enclosing element as being like a block enclosed in these languages by curly braces, the scope rules will seem very familiar.

Two global variables may have the same name only if they have different
import precedence
; in other words, if one of them was in an imported stylesheet (for further details, see

on page 357). In this case, the definition with higher import precedence wins. Note that the higher precedence definition applies everywhere, even within the imported stylesheet that contains the lower precedence definition. This means it is not a good idea to rely on precedence to resolve accidental name clashes between independently developed modules, it's better to use namespaces.

XSLT 1.0 did not allow a local variable to be defined with the same name as another local variable already in scope. This restriction was there as a way of detecting user errors. The restriction has gone in XSLT 2.0, although there is still a warning in the specification advising that this isn't good practice. In fact, not all XSLT 1.0 processors actually enforced this rule. If a variable reference is used when two or more variables with the matching name are in scope, then the reference is taken to refer to the one whose scope is smallest.

This principle extends to variables declared within an XPath expression. To take an extreme example, it is legal to write:

The output of the template will be
A
+
B
+
C
+
D
+
.

These rules on uniqueness and scope of names apply equally to parameters declared using

; the

instruction is effectively just another way of declaring a variable.

The Value of the Variable

The value of the variable may be given either by the XPath expression in the
select
attribute, or by the contents of the contained sequence constructor. If there is a
select
attribute, the

element must be empty. If there is no
select
attribute and the sequence constructor is empty, the value of the variable is a zero-length string, unless there is an
as
attribute, in which case the value is an empty sequence (assuming that the
as
attribute permits an empty sequence).

Figure 6-17
summarizes the different ways of specifying the value. The numbers in brackets refer to the numbered paragraphs below that explain the option in more detail.