[DFDL-WG] DFDL: output and choices/discriminators
Mike Beckerle
mbeckerle.dfdl at gmail.com
Wed Jun 11 15:05:29 CDT 2008
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 at uk.ibm.com]
Sent: Wednesday, June 11, 2008 11:46 AM
To: mbeckerle.dfdl at gmail.com
Cc: dfdl-wg at ogf.org; dfdl-wg-bounces at 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 at uk.ibm.com
Tel: +44 (0)1962 815073 Fax: +44 (0)1962 816898
From:
"Mike Beckerle" <mbeckerle.dfdl at gmail.com>
To:
<dfdl-wg at 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] --
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