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
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"
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>
[attachment "choice-discriminator-example.xml" deleted by Alan
Powell/UK/IBM] --
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