Implicit XPath datatype conversion

No other data types are implicitly converted to node sets. As a consequence, if a node set is expected, you must supply an XPath expression that evaluates to a node set. Otherwise, it's an error.

Whenever a string is expected:

•A node set evaluates to the string-value of the first node in the node set, ie., the node in the node set that is first in document order.

•The number value NaN ("not-a-number") evaluates to the string 'NaN', and other number values convert to the string expression that expresses the number, with any leading zeros and (in decimal numbers) any trailing zeros stripped. Results include the string values:

'10'

'10.5'

'-10'

'0'

•A boolean true() value converts to the string 'true' (the 4 lowercase characters), and a boolean false() value converts to the string 'false' (the 5 lowercase characters).

A number is expected when :

•An XPath expression is passed as an argument to a function that returns a number, eg.,

round( expr )
 

•An XPath expression is passed as a argument in a calculation, eg.:

expr + 5
 

•An XPath expression is involved in a comparison, eg.,:

Whenever a number is expected:

•A node set is first converted to a string, ie., it is converted to the string-value of the node that is the first node in the node set in document order. Then, the resulting string is converted to a number.

•A string converts to a number following the IEEE 754 round-to-the-nearest rule. Any leading zeros and (in decimal numbers) any trailing zeros are first stripped.

•Boolean true() converts to 1 and boolean false() converts to 0.

Whenever a boolean is expected:

1. If a value of a different XPath data type is supplied:

•A non-empty node set evaluates to boolean true(). The empty node set evaluates to boolean false().

•A non-empty string value evaluates to boolean true(). The empty string evaluates to boolean false().
 
NOTE: As an alternative to this, USoft Blend offers "flag-based" string-to-boolean conversion.

•The number values 0 (zero) and NaN (not-a-number) evaluate to boolean false(). Other number values evaluate to boolean true().

2. If a comparison is supplied, that is to say, an XPath expression that contains one of the comparison operators =, !=, <=, <, >, >=, <> :

•If both operands are node sets, the comparison returns true() if there is any node in the first node set and any node in the second node set such that the result of comparing the string-value of the two nodes evaluates to true().

•If one operand is a node set and the other a boolean, the comparison returns true() if the result of converting the node-set to a boolean, when compared to the other boolean, evaluates to true().

•If one operand is a node set and the other a string or number, the comparison returns true() if the result of converting the node-set to a string value, when compared to the other value (string or number), evaluates to true().

•Comparisons between values that are both not a node set (ie., that are both either string, number or boolean) are compared as booleans if at least one of them is boolean; otherwise, as numbers if at least one of them is a number; otherwise, as two strings.