DAP4: DDX Grammar

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Revision as of 22:33, 22 February 2012 by DennisHeimbigner (talk | contribs) (moved DAP4: Grammar to DAP4: DDX Grammar: dyslexia)

Version: 1.0

At the end of this document are instructions for accessing and testing a formal grammar for the DAP4 DDX using the Relax-NG schema language. I constructed it without any reference to any other explicit or implicit grammars so I could record my proposal. I have since modified it based examining the implied grammar in page DAP4: Data Model and from comments from others and from a comparison with the xsd grammar.

1 Differences with DAP4 xsd Grammar

I converted the xsd-based grammar

http://scm.opendap.org/trac/browser/trunk/xml/dap/dap4.xsd

to an equivalent relax-ng grammar.

http://dl.dropbox.com/u/53929684/xsd.rng


One major difference I see is in dimension handling.

  1. I just used the name "dimension" rather than "shareddimension"; For me, all dimensions (except anonymous ones) are shared.
  2. The xsd separates out scalars from arrays. I always allowed the dimensions for a variable to be optional to handle the scalar case.
  3. I attempted to be as consistent as possible, so I allowed any type including sequences and structures to be dimensioned.
  4. The dimensions of a variable are currently specified in the rng grammar as an attribute named "dimensions" associated with "variables": e.g. dimensions="dr d1".
    Previously I used this:
<dimensions>
<dimension name="dr"/>
<dimension name="d1"/>
</dimensions>

But this seemed kind of verbose.

Other differences:

  1. The Dataset element in the dsd has a couple of extra attributes. I added these.
  2. The xsd appears to allow attributes to themselves have attributes. This needs discussion.
  3. I forgot enumerations and opaque. I added them.
  4. The URL basetype is in the xsd. What is the justification for keeping it?
  5. It appears that the Dataset contains a top level <group> declaration; I chose to treat the Dataset itself as the top-level group.
  6. Attribute declarations appear to have their own "namespace" attribute. Not sure why this is needed.
  7. I do not understand the purpose of the "NewAttribute" attribute.
  8. The Grid issue, of course.

There are also some minor differences.

  1. Element names (e.g. <structure>) are capitalized in the xsd grammar.
  2. There is an issue of interleaving of definitions, or equivalently, what elements must occur in a fixed order.
  3. Where should attributes be legal; I think the rng grammar and the xsd grammar agree on this: putting them almost everywhere, but it needs discussion.

Other differences:

  1. I temporarily suppressed OtherXML because it did not translate correctly.
  2. I dropped Blobtype; I fail to see the need for this.

1.1 Testing the Relax-NG Grammar

You will need to copy three files:

  1. dap4.rng - this is the grammar file; it uses the Relax-NG schema language (http://relaxng.org/).
    This can be obtained from http://dl.dropbox.com/u/53929684/dap4.rng
  2. test.xml - this is a test file, that I am growing to cover the whole grammar.
    This can be obtained from http://dl.dropbox.com/u/53929684/test.xml
  3. jing.jar - Jing is a validator that takes the grammar and a test file and checks that the test file conforms to the grammar.
    This can be obtained from http://dl.dropbox.com/u/53929684/jing.jar.

To use it, do the command:

java -jar jing.jar dap4.rng test.xml

No output is produced if the validation succeeds, otherwise, error messages are produced.

-Dennis Heimbigner