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xquery
Ludovic Kuty
Ludovic Kuty

Reputation: 4954

How to check if a variable is defined in XQuery

I'd like to know if it is possible to check the existence of a variable in XQuery. I mean to know if the variable is defined or bound. Some sort of a if (defined($a)).

I have searched on the Internet and a little bit in the XQuery spec without success.

Upvotes: 1

Views: 2210

Answers (2)

Jens Erat
Jens Erat

Reputation: 38712

Variables Are Determined At Compile Time

Undefined variables names are checked in a static analysis phase (let's call it "at compile time" here).

From the W3C XQuery 1.0 Recommendation (similarly applies to XQuery 3.0):

During the static analysis phase, the query is parsed into an internal representation [...]. A parse error is raised as a static error [err:XPST0003]. The static context is initialized by the implementation (step SQ2). [...] The static context is used to resolve schema type names, function names, namespace prefixes, and variable names (step SQ4). If a name of one of these kinds in the operation tree is not found in the static context, a static error ([err:XPST0008] or [err:XPST0017]) is raised [...].

Little Exception: external Variables

The only exception applies to variables defined as external using

    declare variable $var external;

In this case, a dynamic error will be thrown (again cited from the XQuery Recommendation):

A reference to a variable that was declared external, but was not bound to a value by the external environment, raises a dynamic error [err:XPDY0002].

Testing Whether external Variables Are Bound

I'm not aware of a possibility to check at runtime whether an external variable actually is bound, but in XQuery 3.0 you could use try/catch to test this:

xquery version '3.0';
declare variable $var external;

try { $var } 
catch err:XPDY0002
{ "External variable not bound!" }

Upvotes: 2

BeniBela
BeniBela

Reputation: 16917

No, that is not possible.

Accessing an undefined variable is a static error, which is raised during the parsing/compiling of the expression.

Upvotes: 1

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