The main purpose envisaged for fn:exactly-one() was as a guard around off-the-shelf XPath processors. In that scenario, throwing an error is probably required, as what would you do otherwise?

If you wanted to return a true/false value could you just use fn:count($arg) eq 1 instead of fn:exactly-one() ?

Regards

Steve Hanson
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848

From: Mike Beckerle <mbeckerle.dfdl@gmail.com>
To: "dfdl-wg@ogf.org" <dfdl-wg@ogf.org>
Date: 30/10/2014 10:00
Subject: [DFDL-WG] clarification: fn:exactly-one function
Sent by: dfdl-wg-bounces@ogf.org

According to the DFDL spec, fn:exactly-one returns "True if the provided sequence contains exactly one node/value," which suggests that the function would return false if the sequence contains > or < 1 node/value.

The XPath description states that fn:exactly-one "returns $arg if it contains exactly one item. Otherwise, raises an error."

It seems of value to have a true/false result, because something that raises an error like this can only be used as an assertion effectively. On the other hand, that's not what XPath does.

Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology | www.tresys.com
Please note: Contributions to the DFDL Workgroup's email discussions are subject to the OGF Intellectual Property Policy
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