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

[Mike Beckerle]

We implemented casting/conversion based on

https://www.w3.org/TR/xquery-operators/#casting-from-primitive-to-primitive

And I believe these casting/conversions are required for an implementation
of the DFDL expression language, for when sub-expressions are composed with
each other to form larger expressions. E.g., if you call fn:concat, which
takes string arguments, you can pass numbers to it, because they are
auto-converted to strings.

However, the DFDL spec bullets you cite are pretty clear that this
shouldn't be done for the return value of an expression that is providing
the value of a property, like the one for dfdl:textBooleanTrueRep="{ 1*10
}" which should cause an SDE/type-error.

So this is a daffodil bug.

Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology |
www.tresys.com
Please note: Contributions to the DFDL Workgroup's email discussions are
subject to the OGF Intellectual Property Policy
<http://www.ogf.org/About/abt_policies.php>



On Tue, Nov 20, 2018 at 5:37 PM Steve Hanson <smh at uk.ibm.com> wrote:

> 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*
> <http://www.ibm.com/developerworks/library/se-dfdl/index.html>
> Co-Chair, *OGF DFDL Working Group* <http://www.ogf.org/dfdl/>
> *smh at uk.ibm.com* <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* <http://www.tresys.com>
> Please note: Contributions to the DFDL Workgroup's email discussions are
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