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Re: Handling of "error value" in XPATH 2.0

• To: xml-dev@l...
• Subject: Re: Handling of "error value" in XPATH 2.0
• From: "Thomas B. Passin" <tpassin@c...>
• Date: Mon, 13 May 2002 21:00:23 -0400
• References: <004b01c1fa31$e1ba29a0$0bf13044@tbp><200205131521.g4DFLRS29826@d...>

[Joe English]

>
> Thomas B. Passin wrote:
> >
> > I also noticed something else [in XPATH 2.0]
> > and I'd like some assurance that it isn't a problem, or else that it
> > will be handled.
> >
> > I'm referring to "error value", which will sometimes be returned by
XPATH
> > 2.0 expressions.  I haven't found any place in the draft that says
anything
> > about how the "error value" is to be handled.
>
> The use of a distinguished "error value" is a fairly common
> technique in formal semantics; it's used to simplify the definitions.
>
> In the XPATH 2.0 rec, it says:
>
> | _Error_ is a special value indicating that an error has been
> | encountered during the evaluation of an expression. Except
> | as noted in this document, if any operand of an expression
> | is the error value, the value of the expression is also the
> | error value. [2.1 "Basics"]
>

Aha!  I thought I had found every occurrence of "error value", but I must
have had the 1000-yard stare when I got to that one.

> The "Except as noted in this document" clause is the important
> part.  The rest of the spec lists several places where the error
> value may be introduced, but it rarely needs to specify how it's
> handled, because that's covered by this one paragraph.  This
> shortens the spec considerably.
>
> Consider for example 2.5 "Arithmetic Expressions", where there are
> four clauses specfying how an arithmetic expression is evaluated.
> This would easily be twice as long if it explicitly accounted for
> errors at each step.  (By way of contrast, check out the ECMAScript
> standard; nearly every clause in that spec is mostly boilerplate text
> which redescribes how errors are handled.)
>
>
I'm very comfortable with referring to a clause rather than repeating the
boilerplate.
I just want to make sure that it's easy to test for, and that how we test
for error_value won't vary  between implementations (like getting the string
"NaN" - you know what to look for).

Cheers,

Tom P

• References:
  • Handling of "error value" in XPATH 2.0
    • From: "Thomas B. Passin" <tpassin@c...>
  • Re: Handling of "error value" in XPATH 2.0
    • From: Joe English <jenglish@f...>

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