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Re: XQuery needs to prohibit variable redefinition

From: Jonathan Robie <jonathan.robie@datadirect.com>
Date: Thu, 12 Aug 2004 10:55:07 -0400
Message-ID: <411B84CB.6040403@datadirect.com>
To: Per Bothner <per@bothner.com>
CC: public-qt-comments@w3.org

Hi Per,

You are right - this has been discussed several times in the Working 
Group. And you have good instincts in providing us with new evidence for 
topics that have been discussed extensively before.

I am going to forward this to our Working Group list for further discussion.


Per Bothner wrote:

> (I'm told this issue has been brought up before.  However,
> we now have experience: many people *do* make this mistake:
> http://xquery.com/pipermail/talk/2004-July/000272.html
> http://xquery.com/pipermail/talk/2004-July/000273.html )
> Add to 3.8.1 "For and Let Clauses" and 3.11 "Quantified Expressions"
> wording similar to: "A static error is raised if the expanded QName of
> any VarName being bound is the same as the expanded QName of one of
> the existing in-scope variables."
> MOTIVATION 1 (beginners):
> The most common beginner confusion in XQuery is confusion about
> variables, definitions, and assignments.  Here is a typical example:
>   let $x := 0
>     for $y in ...
>       let $x := $x + ...
>         ...
> This kind of code happens because many people new to XQuery are
> confused by the difference between binding and assignment, and
> they're not familiar with hierarchical lexical scoping.
> This is not a hypothetical program: We have all seen people asking why
> this code doesn't work.  When companies deploy products they will
> see many more such support calls.
> A counter-argument (as paraphrased by Dana Florescu): "XQuery is not an
> imperative language and LET is a variable definition, not a variable
> assignment.  It's not by prohibiting small issues like this and
> avoiding this kind of mistake this that will make them understand this
> important fact."  And that is true.  However, if the language prohibits
> redefinition, then the implementation can reject many of the more
> common manifestations of this misunderstanding.  The error message
> can point to a help file which may help them understand the issue,
> If the implementation accepts the erroneous problem, instead people
> get incorrect results, which is a lot harder to understand and track down.
> They will waste more time.  Worse (from an implementor's point of view)
> when they ask for help, the problem is less likely to be obvious
> to figure out than if they ask about a compile-time error message.
> MOTIVATION 2 (everybody):
> Even experienced XQuery users write bugs, and sometimes use the wrong
> variable name.  I have certainly written a loop with an index variable
> that matches a name used in an outer loop.  That is a recipe for
> hard-to-find bugs, so Java wisely added a rule to prohibit such
> redefinition.
> COUNTER-ARGUMENT: Avoid arbitrary restrictions
> Lexical scoping has been used in many languages that allow
> redefinition.  It's easy to understand and consistent. Somebody said
> we don't improve software/languages by adding features, but by
> removing unneeded restrictions - and here I'm arguing for adding an
> unnecessary restriction.
> Perhaps prohibiting redefinition makes XQuery slightly less "elegant"
> in some abstract sense.  However, "elegance" is a secondary goal.  If
> there is no known valid use case for a feature, and it is known that
> programs using it are actually misunderstandings or "think-o"s, and
> that such errors are fairly common, then the feature should go,
> elegance or not.
> Compare the rule ( "Static Analysis phase") "if ... the static
> type assigned to an expression other than () or data(()) is empty(), a
> static error is raised."  This is clearly an arbitrary restriction,
> and the language is simpler and more elegant without this restriction.
> However, this is a case where XQuery has decided it is more useful to
> to catch at compile-time certain otherwise hard-to-detect errors.
> COUNTER-ARGUMENT:  Formal semantics
> The formal semantics does make use of redefined variables.  For
> example, the definitions of path expressions make use of fs:dot, and
> it's difficult to come up with an alternative.  However, there is no
> inherent reason why the core language must be a strict sub-language of
> the surface language: The prohibition of redefinitions can be done at
> or before the rewriting stage, allowing redefinitions in the core
> language.  Alternatively, if you want the core language to be a strict
> sub-language of the surface language, we can specify that the rule
> against redefinition does not apply to a QName in the fake fn:
> namespace.
> I have implemented this prohibition as a warning in my implementation.
> It was not difficult to do so.  I suspect other implementors will do
> the same. I would prefer to change the warning into an error, so I can
> catch this mistake even when warnings are ignored or not shown to the
> programmer.
> For a related comment in the context of XSLT.see:
> http://lists.w3.org/Archives/Public/public-qt-comments/2004Feb/0111.html
Received on Thursday, 12 August 2004 14:55:38 UTC

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