[DFDL-WG] String literal syntax for hexBinary ?? - need DFDL string literal constructor
Steve Hanson
smh at uk.ibm.com
Wed May 23 13:25:20 EDT 2012
Agreed today that such a function is needed, errata taken. Proposals for
function name welcome.
Regards
Steve Hanson
Architect, Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh at uk.ibm.com
tel:+44-1962-815848
From: Steve Hanson/UK/IBM
To: Mike Beckerle <mbeckerle.dfdl at gmail.com>
Cc: dfdl-wg at ogf.org
Date: 20/04/2012 09:04
Subject: Re: [DFDL-WG] String literal syntax for hexBinary ?? -
need DFDL string literal constructor
I think what we need is a function that takes a string and returns the
equivalent DFDL string literal. It looks for the characters that need
cause problems (%, space) and replaces them (with '%%' and '%SP;'
respectively), and if the string is the empty string replaces it with
'%ES;'. I think it's just those that are problematic, as all other
characters will be interpreted correctly (the entity syntax is primarily
just for convenience of data entry).
Regards
Steve Hanson
Architect, Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh at uk.ibm.com
tel:+44-1962-815848
From: Mike Beckerle <mbeckerle.dfdl at gmail.com>
To: Steve Hanson/UK/IBM at IBMGB
Cc: dfdl-wg at ogf.org
Date: 19/04/2012 14:27
Subject: Re: [DFDL-WG] String literal syntax for hexBinary ?? - Re:
String literals - various usage patterns thereof
Ok, these are very good reasons.
So we need the function(s) in our xpath library to make doing this same
substitution easy (e.g., the replace function discussed elsewhere in this
thread would do it I believe because all we have to do is replace "%" with
"%%".)
On Thu, Apr 19, 2012 at 9:20 AM, Steve Hanson <smh at uk.ibm.com> wrote:
The reason that % needs escaping is that most entities start with just % -
eg %NUL; - and it means we have simple rules - you want to use a literal
% then escape it. and if you use just % then we expect to see an entity
next. If we don't do it this way, then we will not be able to extend the
list of entities in the future without breaking existing expressions, and
we won't detect the very common error of leaving off the trailing
semi-colon by mistake..
Regards
Steve Hanson
Architect, Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh at uk.ibm.com
tel:+44-1962-815848
From: Mike Beckerle <mbeckerle.dfdl at gmail.com>
To: Steve Hanson/UK/IBM at IBMGB
Cc: Tim Kimber/UK/IBM at IBMGB, dfdl-wg at ogf.org
Date: 19/04/2012 14:01
Subject: Re: [DFDL-WG] String literal syntax for hexBinary ?? - Re:
String literals - various usage patterns thereof
I don't think % by itself requires any escaping. There is only a need for
escaping when the characters after the % match the syntax for one of our
entities.
I don't expect Dfdl:terminator="%done%" to require any escaping.
On Apr 19, 2012 7:16 AM, "Steve Hanson" <smh at uk.ibm.com> wrote:
Tim, thinking some more on this:
- 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
That is indeed how properties like initiator, terminator and separator
behave today. But there is a problem. Let's say I have dynamically
defined
a separator at the start of my data. The value in the data is %. My
dfdl:separator expression therefore returns %. That will give an error as
a
badly formed DFDL entity. DFDL string literal rules say that you must use
%% to represent a single % character. The expression itself can work
around
this by checking for % and if so substituting %%, but that's a bit
unfriendly especially as fn:replace() is not in the DFDL XPath subset - I
think this is because it comes under this category
http://www.w3.org/TR/xpath-functions/#string.match and not
http://www.w3.org/TR/xpath-functions/#substring.functions. Perhaps we
should include fn:replace() or provide a DFDL function that handles %?
I started wondering whether these properties' expression should return a
list of String. I can envisage no format that has in its data a value that
contains DFDL entity syntax and intends it to mean a DFDL entity! That is
too contrived to be real. However I can certainly envisage an expression
like this:
dfdl:separator = "{if ../version eq 1 then %CR;%LF; else %LF;}"
So I think String is not sufficient and DFDL String Literal must be
allowed.
Regards
Steve Hanson
Architect, Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh at uk.ibm.com
tel:+44-1962-815848
From: Tim Kimber/UK/IBM
To: Steve Hanson/UK/IBM at IBMGB
Cc: Mike Beckerle <mbeckerle.dfdl at gmail.com>, dfdl-wg at ogf.org
Date: 19/04/2012 11:32
Subject: Re: [DFDL-WG] String literal syntax for hexBinary ?? - Re:
String literals - various usage patterns thereof
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 at uk.ibm.com
Tel. 01962-816742
Internal tel. 246742
From: Steve Hanson/UK/IBM
To: Tim Kimber/UK/IBM
Cc: Mike Beckerle <mbeckerle.dfdl at gmail.com>, dfdl-wg at 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 at uk.ibm.com
tel:+44-1962-815848
From: Tim Kimber/UK/IBM
To: Mike Beckerle <mbeckerle.dfdl at gmail.com>
Cc: dfdl-wg at ogf.org, dfdl-wg-bounces at ogf.org, Steve
Hanson/UK/IBM at 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 at uk.ibm.com
Tel. 01962-816742
Internal tel. 246742
From: Mike Beckerle <mbeckerle.dfdl at gmail.com>
To: Steve Hanson/UK/IBM at IBMGB
Cc: dfdl-wg at 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 at 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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--
Mike Beckerle | OGF DFDL WG Co-Chair
Tel: 781-330-0412
Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number
741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU
Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number
741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU
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