I follow your simplification point. This is clever. The point of my example was to motivate how choices work on output though. So the fact that this trick works in this specific case is beside the point I hope. .mike _____ From: Alan Powell [mailto:alan_powell@uk.ibm.com] Sent: Wednesday, June 11, 2008 11:46 AM To: mbeckerle.dfdl@gmail.com Cc: dfdl-wg@ogf.org; dfdl-wg-bounces@ogf.org Subject: Re: [DFDL-WG] DFDL: output and choices/discriminators Mike I think there is an easier solution to your example using an expression for the dfdl:length property of the length field instead of the choice. <xs:element name="OneOrfourByte" type="xs:int" dfdl:alignment="1" dfdl:length='{ if ( ../logicalLength > 127 ) then "31" else "7" }' dfdl:outputValueCalc="{ ../logicalLength }" /> For a simple infoset element which can have multiple physical representations is really support for union which we have excluded from V1. Alan Powell MP 211, IBM UK Labs, Hursley, Winchester, SO21 2JN, England Notes Id: Alan Powell/UK/IBM email: alan_powell@uk.ibm.com Tel: +44 (0)1962 815073 Fax: +44 (0)1962 816898 From: "Mike Beckerle" <mbeckerle.dfdl@gmail.com> To: <dfdl-wg@ogf.org> Date: 09/06/2008 17:42 Subject: [DFDL-WG] DFDL: output and choices/discriminators _____ I'd like to discuss this example in email and/or on our call this week. It illustrates that discriminators must be evaluated both on output and on input. We proposed at the F2F that assertions are only about parsing, but the same cannot be said of discriminators. Because email often line-wraps in ways that break things I've also attached the same example as a file. <?xml version="1.0" encoding="iso-8859-1"?> <!-- 80 column ruler 2345678901234567890123456789012345678901234567890123456 --> <!-- *************************************************************************** **************************************************************************** **** Illustration of use of layering with choices and discrimintors Lessons from this example: Discriminators must be evaluated on output as well as input in order to decide choices. **************************************************************************** **** *************************************************************************** --> <xs:schema targetNamespace=" <http://dataformat.org/tests> http://dataformat.org/tests" elementFormDefault="qualified" xsi:schemaLocation= " <http://www.ogf.org/dfdl/dfdl-0.1> http://www.ogf.org/dfdl/dfdl-0.1 ../../xsd/dfdl.xsd <http://www.w3.org/2001/XMLSchema> http://www.w3.org/2001/XMLSchema ../../xsd/XMLSchema.xsd" xmlns:xs=" <http://www.w3.org/2001/XMLSchema> http://www.w3.org/2001/XMLSchema" xmlns:xsi=" <http://www.w3.org/2001/XMLSchema-instance> http://www.w3.org/2001/XMLSchema-instance" xmlns=" <http://www.ogf.org/dfdl/tests> http://www.ogf.org/dfdl/tests" xmlns:dfdl=" <http://www.ogf.org/dfdl/dfdl-0.1> http://www.ogf.org/dfdl/dfdl-0.1"> <!-- ************************************************************************* Our default format will be binary, without delimiters, with bits for alignment units, and implicit length kind which we'll override where needed. ************************************************************************* --> <xs:annotation><xs:appinfo source=" <http://www.ogf.org/dfdl/dfdl-0.1> http://www.ogf.org/dfdl/dfdl-0.1"> <dfdl:defineFormat name="default"> <dfdl:format representation="binary" lengthKind="implcit" initiator="" separator="" terminator="" alignmentUnits="bits" /> </dfdl:defineFormat> </xs:appinfo></xs:annotation> <!-- ************************************************************************* Example: a string with smart one byte or four byte length preceding it. Modeled as a single bit flag, followed by a 7 bit or 31 bit integer after it. ************************************************************************* --> <xs:complexType name="smartLengthString" dfdl:ref="default" dfdl:lengthKind="explicit" dfdl:lengthUnits="bits"> <xs:sequence> <xs:annotation><xs:appinfo source="..."> <dfdl:hidden> <xs:element name="lengthFlag" type="xs:byte" dfdl:length="1" dfdl:alignment="8" dfdl:outputValueCalc='{ if ( ../logicalLength > 127 ) then "1" else "0" }' /> <xs:choice dfdl:choiceKind='variable' dfdl:choiceResolvable="true"> <!-- First choice alternative: one byte --> <xs:sequence> <xs:annotation><xs:appinfo source=" <http://www.ogf.org/dfdl/dfdl-0.1> http://www.ogf.org/dfdl/dfdl-0.1"> <dfdl:discriminator test="{ ../lengthFlag != '1' }" /> </xs:appinfo></xs:annotation> <xs:element name="oneByte" type="xs:byte" dfdl:alignment="1" dfdl:length="7" dfdl:outputValueCalc="{ ../logicalLength }" /> </xs:sequence> <!-- Second choice alternative: one byte --> <xs:element name="fourByte" type="xs:int" dfdl:alignment="1" dfdl:length="31" dfdl:outputValueCalc="{ ../logicalLength }" /> </xs:choice> <!-- this logicalLength element below isn't strictly speaking needed in this example. It's here to illustrate something having both input and output value calculation, and makes things a bit more readable. --> <xs:element name="logicalLength" type="xs:int" dfdl:inputValueCalc="{ if (../lengthFlag = '1') then ../fourByte else ../oneByte }" dfdl:outputValueCalc= "{ dfdl:length(../str, 'characters') }" /> </dfdl:hidden> </xs:appinfo></xs:annotation> <xs:annotation><xs:appinfo source="..."> <xs:element name="str" type="xs:string" dfdl:length="{ ../logicalLength }" dfdl:lengthUnits="characters" /> </xs:sequence> </xs:complexType> </xs:schema> [attachment "choice-discriminator-example.xml" deleted by Alan Powell/UK/IBM] -- dfdl-wg mailing list dfdl-wg@ogf.org <http://www.ogf.org/mailman/listinfo/dfdl-wg> http://www.ogf.org/mailman/listinfo/dfdl-wg _____ Unless stated otherwise above: IBM United Kingdom Limited - Registered in England and Wales with number 741598. Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU