[DFDL-WG] clarification needed: conversions to required type of expressions - implicit or explicit

[Steve Hanson]

These are the relevant bullets from section 23.3:

„h      Every property that accepts an expression states exactly what the 
expression is expected to return. To ensure the returned value is of the 
correct type, an expression must use XPath constructors or the correct 
literal values. 
„h      No extra auto-casting is performed over and above that provided by 
XPath 2.0. XPath 2.0 has rules for when it promotes types and when it 
allows types to be substituted. These are in Appendix B.1 of the XPath 2.0 
spec.

Note the use of 'must' in the first bullet.  I don't see anywhere in XPath 
2.0 spec that covers your auto-casting example. Promoting int to string is 
not covered by XPath 2.0 spec Appendix B.1 as far as I can see.

I think I can recall this being discussed way back, and the conclusion was 
to always use constructors to make expressions explicit.
 
Regards
 
Steve Hanson
IBM Hybrid Integration, Hursley, UK
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
smh at uk.ibm.com
tel:+44-1962-815848
mob:+44-7717-378890
Note: I work Tuesday to Friday 



From:   Mike Beckerle <mbeckerle.dfdl at gmail.com>
To:     DFDL-WG <dfdl-wg at ogf.org>
Date:   20/11/2018 18:22
Subject:        [DFDL-WG] clarification needed: conversions to required 
type of expressions - implicit or explicit
Sent by:        "dfdl-wg" <dfdl-wg-bounces at ogf.org>



We have a test for Daffodil that does this:

<element name="y" type="xsd:boolean" maxOccurs="unbounded"
      dfdl:textBooleanTrueRep="{ 1*10 }" dfdl:textBooleanFalseRep="{'a b 
c'}" />

We found that we must change this to:

      dfdl:textBooleanTrueRep="{ xsd:string(1*10) }"

For the test to be portable and work on IBM DFDL as well as Daffodil. 

So the issue to discuss is, are string conversions, or in general 
conversions to the required property's type (e.g., integer for 
dfdl:length), are these implicit, or do they have to be explicit?

I didn't find language in the DFDL spec that makes this clear.  The 
properties tend to be clear on what the type needs to be (string or 
integer typically). 

The XPath language that is what we derive DFDL's expression language from, 
has lots of conversion/coercion rules. Everything can be implicitly 
converted to a string.

We say our DFDL expression language is a subset of XPath 2.0, and that we 
explicitly say we use XPath 2.0's type system and respect its errors, and 
we say DFDL implementations may use XPath 2.0 implementations if they do 
some pre-processing. 

So the clarification is reduced to this: Does an entire DFDL expression 
for the value of a property behave as if it was an actual argument to a 
function, that formal argument having the type required by the property? 
In that case coercions would be inserted to convert say, numbers to 
strings.

Alternatively, does the expression for a property have to explicitly 
convert to the type that the property requires?

Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology | 
www.tresys.com
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