Please help me understand what you need better.
As co-chair on the DFDL workgroup, if there is something you need to be able to
express, and there truly is no good way to do it, then I would consider that a
DFDL flaw that we should really examine. If we (by we, I mean the DFDL
workgroup, and the DFDL user community) decide it's a flaw, we would propose
and implement a fix in Daffodil, then propose it for inclusion in the DFDL
standard. So even if DFDL can't do something today, doesn't mean it wouldn't
get fixed fairly quickly in Daffodil.
You are looking for abstract types. I'm not sure what you are trying to be
abstract about.
One can write an element, giving only where the length comes from.
<xs:element name="data" type="tns:dataN"
dfdl:length="{../../parameterLength}"/>
To me, dataN here is an abstract type where you must provide length property to
make it concrete. All the other properties are fixed in the type definition, or
in the lexical environment (top level format) surrounding that type definition.
You can even make it reusable with the length coming from a known relative
location.
<xs:simpleType name="dataParameterizedLength"
dfdl:length="{../../parameterLength}">
<xs:restriction base="tns:dataN"/>
</xs:simpleType>
<xs:element name="data" type="tns:dataParameterizedLength"/>
In this case the type "dataParameterizedLength" is a concrete type, where dataN
is the abstract base.
You or you can use variables to guide whether the length is parameterized or
fixed:
<xs:simpleType name="dataParameterizedLength"
dfdl:length="{ if ($tns:useFixedLengths) then 1
else ../../parameterLength }">
<xs:restriction base="tns:dataN"/>
</xs:simpleType>
What you can't do is define a type completely with all representation types,
and then evolve them and change your mind in derived types. You have to
anticipate the need to override, and create an abstract type that lacks the
properties you want to specify separately.
________________________________
From: Christofer Dutz <[email protected]>
Sent: Wednesday, May 8, 2019 11:39 AM
To: [email protected]
Subject: Re: Some sort of "abstract" types?
Hi Mike,
but if I want to read a number of bytes which size is defined by another
property, then things get complicated/impossible.
I am currently trying to separate the reading of bits/bytes which is sort of
language dependent into a language-dependent IO module
and have the structure of the message use these globally defined types to
generate the datatypes and the parsers/serializers.
So for now I’ll probably have to give up the ability to run my schemas in
daffodil …
I would have liked to continue as I really liked things like the test-suite
stuff and such, but right now it’s this which is causing trouble …
perhaps we’ll come up with exceptions for hexByte and string and will allow
only there to be defined inline and force the rest to be imported.
Chris
Von: "Beckerle, Mike" <[email protected]>
Antworten an: "[email protected]" <[email protected]>
Datum: Mittwoch, 8. Mai 2019 um 17:18
An: "[email protected]" <[email protected]>
Betreff: Re: Some sort of "abstract" types?
Alas, the DFDL Workgroup made a "conservative design choice" a long while back
to disallow overlapping properties between referencer and referencee, where
those referencing scenarios are:
element -> type def
elementRef -> element decl
groupRef -> model group of group def
simple type def -> simple base type def
The rationale for this was that we could always loosen this restriction in the
future, but some of the committee members believed this restriction would
simplify implementation complexity.
To achieve roughly what you want you have to do:
<xs:simpleType name="dataN" dfdl:lengthUnits="bytes"
dfdl:lengthKind="explicit">
<xs:restriction base="xs:hexBinary"/>
</xs:simpleType>
<xs:simpleType name="data1" dfdl:length="1">
<xs:restriction base="tns:dataN"/>
</xs:simpleType>
Then at your points of use of the type, you use data1 or dataN accordingly.
________________________________
From: Christofer Dutz <[email protected]>
Sent: Wednesday, May 8, 2019 10:36:34 AM
To: [email protected]
Subject: Some sort of "abstract" types?
HI,
I am currently working on a first code-generator for my DFDL schemas … as
implementing the full spec is completely out of question as it’s just far to
feature-rich and hereby complex to implement, I am currently refactoring my
schemas to allow everything I need, but as simple as possible. While I was able
to come up with solutions for most things, one thing is a little annoying:
We have an element like this:
<xs:element name="data" type="xs:hexBinary"
dfdl:byteOrder="bigEndian" dfdl:lengthUnits="bytes"
dfdl:lengthKind="explicit"
dfdl:length="{../../parameterLength}"/>
We moved the definition of all types to the top in “simpleType” elements and
simply reference “plc4x:uint8” for example, however we can’t do that with these
as we can’t override the “length” later on …
I would have preferred something like this:
<xs:simpleType name="data" dfdl:lengthUnits="bytes" dfdl:length="1"
dfdl:lengthKind="explicit">
<xs:restriction base="xs:hexBinary"/>
</xs:simpleType>
And then override the length where needed:
<xs:element name="data" type="plc4x:data"
dfdl:length="{../../parameterLength}"/>
Are there any options to do such a thing and I just haven’t found them yet?
Chris
