[DFDL-WG] String literal syntax for hexBinary ?? - Re: String literals - various usage patterns thereof

[Mike Beckerle]

On Thu, Apr 19, 2012 at 4:35 AM, Tim Kimber <KIMBERT at 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.

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.

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.

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'.

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.

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.

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.

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.

Case X4.5

Ouch check this out:

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

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.)

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.

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.

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.)

Rule 2: in a DFDL xpath expression that returns a string value
(inputValueCalc - is this the only case?) the value is not examined for
DFDL entities.

Rule 3: dfdl:property returns the value of a property before any DFDL
entities replacements have been done.

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

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.

- 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.


> 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.

...mikeb
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