I agree with all of that. One refinement, though. I don't think it's necessary to *require* an implementation to auto-cast the result of a DFDL Expression into the target type. If an implementation wants to be picky about the return type AND issue a clearly-worded Schema Definition Error stating what the problem is then I think we should allow it.

Arguably, this would reduce the portability of DFDL schemas, but there is precedent for defining a portable subset of a language while allowing conveniences for users who don't need portability ( e.g. ANSI 'C' ). We already take that line for the regular expression syntax, so there is precedent for this in the DFDL specification too.

regards,

Tim Kimber, Common Transformation Team,
Hursley, UK
Internet: kimbert@uk.ibm.com
Tel. 01962-816742
Internal tel. 246742

From: Steve Hanson/UK/IBM
To: Tim Kimber/UK/IBM
Cc: Mike Beckerle <mbeckerle.dfdl@gmail.com>, dfdl-wg@ogf.org
Date: 19/04/2012 11:04
Subject: Re: [DFDL-WG] String literal syntax for hexBinary ?? - Re: String literals - various usage patterns thereof

Hi Tim

Firstly, both your bulleted assertions are correct, but your conclusion is not.

Secondly, let me flesh out my earlier reply about constructor functions.

The last paragraph of Section 23 says: "The result of evaluating the expression must be a single atomic value of the type expected by the context, and it is a schema definition error otherwise".

This is where the XPath constructors come into play. Eg: <element name="myHexBin" type="xs:hexBinary" dfdl:inputValueCalc="{ xs:hexBinary(...) }"/>
These xs: constructors, plus the special fn:dateTime() constructor that DFDL adds, allow the correct types to be created.

Note that you don't always need the constructors. An expression that returns a quoted value is returning an XPath string literal so that is automatically xs:string. An expression that returns an unquoted number is returning an XPath number literal so that can be xs:decimal, xs:integer, xs:double (depends whether the number contains a '.' or 'e' or 'E').
This is described here: http://www.w3.org/TR/xpath20/#id-literals

So simply returning the literal 'DEADBEEF' will return an xs:string and if the context requires xs:hexBinary that is a schema definition error according to DFDL spec.

A clarification is worth while though. Take the following expression: {if ../type eq 'A' then 10000 else 20000}. That returns xs:integer.
- What if my context was xs:decimal? xs:integer is a restriction of xs:decimal so the value will always be in range, so is that 'auto-cast' allowed?
- What if my context was xs:long or another restriction of xs:integer? The value may or may not be in range, so is that 'auto-cast' iff value in range?
I think that we should auto-cast when type restrictoions are involved, and clarify that in the spec.

We *could* change the spec to say that the result of the expression is always automatically cast to the type expected by the context. That takes some of the burden off the modeler and makes it much more likely that expressions written by XPath novices will return the correct results. But it could also hide accidental errors. Note this proposal I shall call (d) as it not the same as Mike's (c). If we made this change, then returning the literal 'DEADBEEF' for xs:hexBinary would succeed. I don't think it affects the desire for expressions to be statically type checkable - because it is known whether type X can be cast to type Y, so a cast mismatch can be statically detected.

Regards

Steve Hanson
Architect, Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848

From: Tim Kimber/UK/IBM
To: Mike Beckerle <mbeckerle.dfdl@gmail.com>
Cc: dfdl-wg@ogf.org, dfdl-wg-bounces@ogf.org, Steve Hanson/UK/IBM@IBMGB
Date: 19/04/2012 09:35
Subject: Re: [DFDL-WG] String literal syntax for hexBinary ?? - Re: String literals - various usage patterns thereof

I'm pretty sure that the rules are:
- DFDL expressions must not *contain* DFDL String Literals. They must be valid XPath 2.0 expressions except that the list of allowable function names includes the DFDL extension functions.
- A DFDL expression is sometimes allowed to *return* a DFDL String Literal. In this case, the returned value is an xs:string that conforms to the DFDL String Literal syntax. But that does not apply to your example because the dfdl:inputValueCalc must return a value ( an XML value ) that is valid for the type of the element.

I think that corresponds to your answer a) ; 'DEADBEEF' is a valid xs:hexBinary lexical value.

regards,

Tim Kimber, Common Transformation Team,
Hursley, UK
Internet: kimbert@uk.ibm.com
Tel. 01962-816742
Internal tel. 246742

From: Mike Beckerle <mbeckerle.dfdl@gmail.com>
To: Steve Hanson/UK/IBM@IBMGB
Cc: dfdl-wg@ogf.org
Date: 19/04/2012 07:42
Subject: [DFDL-WG] String literal syntax for hexBinary ?? - Re: String literals - various usage patterns thereof
Sent by: dfdl-wg-bounces@ogf.org

What is the DFDL string literal syntax for a hexBinary type value?

E.g., I want a hex binary whose value is the 4 bytes described by this hex: DE AD BE EF.

<element name="myHexBin" type="xs:hexBinary" dfdl:inputValueCalc="{ ... }"/>

So, what can one syntactically put, for literal constant values, in the input value calculation expression?

Note that this is legal pure (non-DFDL) XSD (I think)

<element name="aHexBin" type="xs:hexBinary" fixed="DeadBeef"/>

That is, the fixed/default are allowed and one specifies these values as just strings of hex digits. Notice no special escaping or anything. You just use a string that just so happens to contain hex digits.

I think there are three possibilites
(a) we allow "DEADBEEF" i.e., because the type of the expression is hexBinary, a string is cast to hexBinary by interpreting it as hex nibbles.
(b) we require a special kind of string literal - a bytes-only string literal, so for example: "%#rDE;%#rAD;%#rBE;%#rEF;" is the way you create 4 bytes. If you just put characters, then that's a processing error - like a cast failure. Only raw-bytes allowed.
(c) Anything you return from the expression is converted to a hexBinary by unparsing it to bytes (using current properties), then using the bytes as the hexBinary data. So you could have an expression that returns a double, and that would create 8 bytes if representation="binary". In this case the decimal number 3735928559 (hex 0xdeadbeef) as a binary bigEndian int would produce the 4 bytes I want.

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