XPath and XQuery Functions and Operators 4.0

17.1.4 fn:function-identity

Summary

Returns a string representing the identity of a function item.

Signature

fn:function-identity(
$function	as fn(*)
) as xs:string

Properties

This function is deterministic, context-independent, and focus-independent.

Rules

The fn:function-identity function returns a string that represents the identity of $function.

The returned string has the property that fn:function-identity($f1) and fn:function-identity($f2) are codepoint-equal if and only if $f1 and $f2 have the same function identity. Apart from this property, the result is implementation-dependent.

Any label attached to a function item is ignored (see Section 3.3 Labeled Items^DM). Specifically, if L is a labeled item then fn:function-identity(L) returns the function identity of the subject of L.

In the case of maps and arrays, the result follows the following rule: If $X are both maps or arrays then fn:function-identity($X)must not be codepoint-equal to fn:function-identity($Y) unless $X and $Y are indistinguishable, that is unless every operator or function applied to $X returns the same result as for $Y. Even in this case, however, the result of the comparison fn:function-identity($X) eq fn:function-identity($Y) is implementation-dependent.In the case of maps and arrays, the result follows the following rule: If $X and $Y are both maps or arrays then fn:function-identity($X)must not be codepoint-equal to fn:function-identity($Y) unless $X and $Y are indistinguishable, that is unless every operator or function applied to $X returns the same result as for $Y. Even in this case, however, the result of the comparison fn:function-identity($X) eq fn:function-identity($Y) is implementation-dependent.In the case of maps and arrays, the result follows the following rule: If $X and $Y are both maps or arrays then fn:function-identity($X)must not be codepoint-equal to fn:function-identity($Y) unless $X and $Y are indistinguishable, that is unless every operator or function applied to $X returns the same result as for $Y. Even in this case, however, the result of the comparison fn:function-identity($X) eq fn:function-identity($Y) is implementation-dependent.

Notes

This function enables applications to test whether two expressions or variables reference the same function item. This may be useful, for example, to allow caching of function results to avoid repeated evaluation. The results of previous function invocations might be held in a map whose key is the function identity.

The function identity, by definition, is generated upon the creation of a function item. Specific expressions that create function items have their own rules for the identity of the returned functions: for example, it is guaranteed that evaluation of a function reference to a system function with no captured context (such as fn:abs#1) will always return the same function item.

It is not meaningful to store or compare the result of calling fn:function-identity across different execution scopes, because the string used to represent the function identity will generally vary from one execution scope to another.

The result of an expression such as function-identity(abs#1) eq function-identity(abs(?)) may be either true or false, because it is implementation-dependent whether abs#1 and abs(?) return the same function item.

Similarly, function-identity({1:()}) eq function-identity(map:entry(1, ())) may be either true or false.

Labels on function items are ignored because they typically represent information about how the function item was retrieved, rather than about the item itself. For example, a function item held in a map might be retrieved using a variety of lookup expressions, which may return the same function item but with different labels.

Examples

Expression	Result
function-identity(abs#1) eq function-identity(abs#1)	true()
function-identity(abs#1) eq function-identity(round#1)	false()
function-identity({1:0}) eq function-identity({1:1})	false()
function-identity([0]) eq function-identity([1]})	false()