XPath and XQuery Functions and Operators 4.0

17.3.4 fn:op

Summary

Returns a function whose effect is to apply a supplied binary operator to two arguments.

Signature

fn:op(
$operator	as xs:string
) as fn(item()*, item()*) as item()*

Properties

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

Rules

The supplied operator must be one of:

",", "and", "or", "+", "-", "*", "div", "idiv", "mod", "=", "<", "<=", ">", ">=", "!=", "eq", "lt", "le", "gt", "ge", "ne", "<<", ">>", "precedes", "follows", "is", "is-not", "||", "|", "union", "except", "intersect", "to", "otherwise"",", "and", "or", "+", "-", "*", "div", "idiv", "mod", "=", "<", "<=", ">", ">=", "!=", "eq", "lt", "le", "gt", "ge", "ne", "<<", ">>", "precedes", "follows", "is", "is-not", "||", "|", "union", "except", "intersect", "to", "otherwise".",", "and", "or", "+", "-", "*", "div", "idiv", "mod", "=", "<", "<=", ">", ">=", "!=", "eq", "lt", "le", "gt", "ge", "ne", "<<", ">>", "precedes", "follows", "is", "is-not", "||", "|", "union", "except", "intersect", "to", "otherwise".

The result of calling fn:op("⊙"), where ⊙ is one of the above operators, is the function represented by the XPath expression:

fn($x, $y) { $x ⊙ $y }

For example, op("+") returns fn($x, $y) { $x + $y }.

Error Conditions

A type error is raised [err:XPTY0004]^XP if the supplied argument is not one of the supported operators.

Notes

The function is useful in contexts where an arity-2 callback function needs to be supplied, and a standard operator meets the requirement.

For example, the XSLT xsl:map instruction has an on-duplicates attribute that expects such a function. Specifying on-duplicates="op(',')" is equivalent to specifying on-duplicates="fn($x, $y) { $x, $y }

The function is also useful in cases where the choice of operator to apply is made dynamically.

Some operators (such as and, or, and otherwise) have custom error handling semantics, with the effect that evaluating one of the operands cannot cause an error unless the other operand has a particular value (see Section 2.4.5 Guarded Expressions^XP). Although implementations are free to make optimizations, it should be assumed that a function call such as op('and')(X, Y) will have the normal semantics of a dynamic function call, where the arguments are evaluated in any order, and a failure evaluating any argument may cause the function call as a whole to fail.

Examples

Expression	Result
for-each-pair(21 to 25, 1 to 5, op("+"))	22, 24, 26, 28, 30
for-each-pair(21 to 25, 1 to 5, op("-"))	20, 20, 20, 20, 20