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"

           elementFormDefault="qualified"

           xsi:schemaLocation=

               "http://www.ogf.org/dfdl/dfdl-0.1 ../../xsd/dfdl.xsd

                http://www.w3.org/2001/XMLSchema ../../xsd/XMLSchema.xsd"

           xmlns:xs="http://www.w3.org/2001/XMLSchema"

           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

           xmlns="http://www.ogf.org/dfdl/tests"

           xmlns:dfdl="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">

<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">

              <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>