I have responded in-line. Agree with nearly everything.

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: Mike Beckerle <mbeckerle.dfdl@gmail.com>
To: Tim Kimber/UK/IBM@IBMGB
Cc: dfdl-wg@ogf.org
Date: 19/04/2012 16:07
Subject: Re: [DFDL-WG] String literal syntax for hexBinary ?? - Re: String literals - various usage patterns thereof
Sent by: dfdl-wg-bounces@ogf.org

On Thu, Apr 19, 2012 at 4:35 AM, Tim Kimber <KIMBERT@uk.ibm.com> wrote:

- 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.

I'm pretty sure the above statement isn't right, or doesn't mean to me what you intended.
SMH: I read this as meaning that DFDL string literals do not make sense within the expression, only when you interpret the value returned by the expression. I agree with that.

Some expressions return string literals, and so their component parts must be able to contain string literal syntax or fragments thereof. What we don't want is for the semantics to require that those string literal syntaxes be interpreted by the xpath processor.

Let me analyze this by cases. Below are what I think are the right behaviors.

Case X1:

Appearing in dfdl:initiator="{ '%#234;' } The result for the initiator is one character, exactly as if one had written dfdl:initiator="%#234;" That is, the return value of the expression is then subsequently treated as a string literal. So I could also return a whitespace separated list of initiators if I wanted to.
SMH: Agree

The implications of this are that a few things one might want to return from an expression will cause issues. Ex: suppose dfdl:separator="{...}" and the expression wants to return a space character. In that case one must check for that and return "%SP;" instead. Whitespace generally will cause issues. Similarly "%" has to be "%%". This is a headache, but I feel it is preferable to having different sets of rules for expression and non-expression cases. Doing this escapifying does require a replace function on strings, as has been pointed out elsewhere. Just a basic replace might not be sufficient. We might want a dfdl:escapify(...) function to deal with the all-varieties-of-whitespace issue.
SMH: Agree. Yes it makes some things awkward and we need to make it easier to work round.

Case X2:
Appearing in dfdl:initiator="{ fn:concat('%#23', '4;') }" also represents one character, as it is the result of the xpath evaluation that we analyze to see what it means.

I'm expecting this to be controversial. But again it is the result of the expression that is a string 'literal'.
SMH: Agree

Case X2.5:

Suppose I have a header field. If the value is N, it means terminator is ASCII null. So I want to write

dfdl:terminator="{ if (headerIndicator = 'N') then '%NUL;' else ';' }"

In that case I really do need to post process the expression to find the %NUL; and convert to a zero codepoint value. I can't see any other way to get the zero codepoint into the terminator expression in this case. This case X2.5 doesn't introduce anything new, it's just amplifying the point of case X2.
SMH: Agree

Case X3:

Appearing in <element name="foo" type="xs:string" dfdl:inputValueCalc="{ '%#234;' }"/>

I am pretty sure this is 6 characters. It's a string value. There is nothing said about string literals here.
SMH: Agree

Case X4:

Appearing in <sequence dfdl:separator="{ if ('%#x2c;' = ',') then ';' else '!' }">....</sequence>

The above would appear to need to interpret the dfdl string literals as soon as they are created down within the expression. That is the right thing, but I suggest we could live without this.
SMH: We should not interpret DFDL string literals in the middle of expressions. Which is the position you take in Rule 3.

We need to be very clear if we want to say only the result of an Xpath is ever interpreted for dfdl entities and then only for certain properties.
SMH: We should say this clearly. The spec implies this today.

Case X4.5

Ouch check this out:

<sequence dfdl:initiator="{ '%#x2c;' }" dfdl:terminator="," dfdl:separator="{ if (dfdl:property('initiator', '.') eq dfdl:property('terminator', '.')) then ';' else '!' }"> .... </sequence>
SMH: Syntax corrected to add 2nd argument (path)

Does dfdl:property return the value after or before entities have been replaced?

I'm assuming here it returns the "value" of the property, i.e., any expressions have been evaluated. But has the entity substitution been done?

I believe the right answer here is that the value of the property is the value before DFDL entities have been replaced. That prevents a referential transparency gap, and a bunch of totally bizarre stuff like people using delimiters just to get the entities substitution done, asking for the value of them with dfdl:property(...), and then redefining the delimiter back to say "". (Basically, we want to avoid exposing the implementation's entity processing behavior as a user-visible behavior.)
SMH: Agree. But are we all happy that the result of dfdl:property when the property value is an expression, is the result of executing that expression? How complicated is that going to make implementations?

Case X5:

Appearing in <element name="bar" type="xs:string" default="{ '%#234;' }"/> it's 12 characters, because it's not even an expression when it appears in XSD string literal context.

I'm not expecting any controversy here. This seems weird, but it is part of being embedded properly in XSD.
SMH: Agree.

Summary:

I think there are rules we need to articulate.

Rule 1: if a DFDL property takes an expression in addition to other literal syntax (enum, or string literal of some kind), then the expression can return a string containing the same syntax as the enum or string literal that the property accepts, and it is interpreted the same way.
SMH: Agree. And all property descriptions in the spec say this, I believe.

We do have one exception to this already unfortunately, which is we don't allow an expression to return "" in case of delimiters (thereby dynamically shutting off the use of the delimiter).

(Side note: I no longer require this restriction. I asked for this, and I still think it's probably a good idea, but my concern when I asked for this restriction was based on implementation concerns. Much more implementation thought has gone into this now, and the planned implementation technique can handle this, so I don't see a requirement here anymore. Apologies for flip-flopping on this issue.)
SMH: We should keep this restriction please.

Rule 2: in a DFDL xpath expression that returns a schema typed value (inputValueCalc - is this the only case?) the value is not examined for DFDL entities.
SMH: Agree, with correction. Also outputValueCalc (type of element), discriminators and asserts (boolean), defineVariable, newVariableInstance, setVariable (type of variable).

Rule 3: dfdl:property returns the value of a property before any DFDL entities replacements have been done.
SMH: Agree. But I think we should discuss whether it evaluates an expression.

So dfdl:textStandardDecimalSeparator="{ fn:concat('%#x2', 'c;') }" works, creates a %#x2c; which is the codepoint for a comma I believe.
SMH: Agree.

but... dfdl:textStandardDecimalSeparator="{ if (fn:concat('%#x2', 'c;') = ',') then ',' else ' %SP;' }" the predicate fails because the intermediate result of the concat is not examined for DFDL entities, so the result is %SP;. That entity is however interpreted correctly as a space character because the final result of the expression IS examined for entities.
SMH: Agree.

- 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.

Agreed. I had to argue myself into it, but I do think this is right now.
SMH: Agree.

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

This issue seems orthogonal to me now. I do agree that if XSD allows "DEADBEEF" as a literal for the default value of a hexBinary, then DFDL expressions should do the same.
SMH: Disagree. XPath 2.0 does not have the same rules as XSDL in this regard. I specifically checked this point.

...mikeb-- dfdl-wg mailing list dfdl-wg@ogf.org https://www.ogf.org/mailman/listinfo/dfdl-wg

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