RE: XQuery static types, document() and path expressions

Oops - slight error below.  In Example 3 I claimed that document() is
type item*, when I knew all along it is node*.  This doesn't really
change anything.  In fact, it means that Example 3 is now worse - we do,
in fact, get a static type of (), and then a dynamic evaluation of a
sequence of <bib> nodes.

 

            -Todd

 

-----Original Message-----
From: public-qt-comments-request@w3.org
[mailto:public-qt-comments-request@w3.org] On Behalf Of Todd A. Mancini
Sent: Wednesday, April 16, 2003 10:09 PM
To: public-qt-comments@w3.org
Subject: XQuery static types, document() and path expressions

 

I've been trying to make sense of some of the static typing issues
related to path expressions.  Perhaps someone can point me to the
correct answers (or let me know that some of these are still under
discussion).

 

EXAMPLE 1:

 

Let's warm up with an easy example that does not involve paths.  This
will lay the groundwork for the next example, unless this example is
completely wrong.

 

let $a := <foo><bar/></foo> return $a

 

Is it correct to say that the static type of this expression is

 

element foo of type xs:anyType

 

(Note I have not imported any schemas, so <foo> was not defined before
this expression was evaluated.)

 

I deduced this static type based upon Formal Semantics 5.7.2.4 and prior
discussions regarding global and local elements, whereby the content
model of element foo is statically typed to xs:anyType even though the
element bar is 'just sitting there', waiting to be used.

 

EXAMPLE 2:

 

            let $a := <foo><bar/></foo> return $a/child::bar

 

Is this a static type error?  Does the child::bar StepExpr have nothing
valid to match again w.r.t static typing?  If my 1st example was
correct, then I believe this example is a static type error.  $a would
have static type element foo of type xs:anyType, and bar will fail to
match to xs:anyType (at least, that's how I read the judgments).  Does
there need to be a judgment or two that allows for broader matching?
Should we have avoided xs:anyType in the first place and had element bar
as the content model of element foo?  Or am I just reading the spec.
incorrectly?  (If I have correctly interpreted this to be a static type
error and the desire is to not have this be a type error, then I vote
for replacing xs:anyType with element bar when computing the static type
of <foo><bar/></foo>.  I know there are open issues regarding this; I
eagerly await the next draft to see if this has been resolved.)

 

EXAMPLE 3:

 

            document("bib.xml")/bib

 

Is this a static error?  The only thing we statically know about
document("bib.xml") is that it has static type item*, and I don't see
any judgments which properly match the static typing to return anything
other than an error (or perhaps type () ).  In fact, returning type ()
would be even worse, because if dynamic evaluation were to return a
sequence of <bib> elements then we'd have a mismatch between static type
analysis and dynamic evaluation.  (I actually think we get an error and
not (), because () would result from the 'catch-all' judgment:

 

 <http://www.w3.org/TR/query-semantics/#xq_stat_env_def#xq_stat_env_def>
statEnv |- PrincipalNodeKind, node()
<http://www.w3.org/TR/query-semantics/#jd_type_filter_on#jd_type_filter_
on> on NodeType
<http://www.w3.org/TR/query-semantics/#jd_type_filter_on#jd_type_filter_
on> : ()

 

and item does not match node().  So there are no matching judgments,
which I assume to imply static type error.)

 

It seems to me that this expression should evaluate meaningfully, and
that requires that it not be a static type error or type ().  Note that
if the static type of the function document() is changed to be document,
things are actually worse.  In this case, we do match the catch-all
judgment and report a static type of ().  We then potentially return a
sequence of bib elements, which fails to match our statically computed
type.  The system is inconsistent; it can probably be fixed by
introducing some additional judgments concerning type document.

 

EXAMPLE 4:

 

            document("bib.xml")/*

 

This does not seem to me to match any judgment at static type check
time.  The only judgment that comes close is

 

 <http://www.w3.org/TR/query-semantics/#xq_stat_env_def#xq_stat_env_def>
statEnv |- PrincipalNodeKind, node()
<http://www.w3.org/TR/query-semantics/#jd_type_filter_on#jd_type_filter_
on> on NodeType
<http://www.w3.org/TR/query-semantics/#jd_type_filter_on#jd_type_filter_
on> : ()

 

But even this does not match, as document() returns item*, not node*,
Am I missing a matching judgment?  I do not feel that 

 


  _____  


 <http://www.w3.org/TR/query-semantics/#xq_stat_env_def#xq_stat_env_def>
statEnv |- element, *
<http://www.w3.org/TR/query-semantics/#jd_type_filter_on#jd_type_filter_
on> on element prefix:local {Type}
<http://www.w3.org/TR/query-semantics/#jd_type_filter_on#jd_type_filter_
on> : element prefix:local {Type} 

 

 

is a matching judgment because document() has static type item*, and we
therefore cannot compute a meaningful NodeType.

 

I think this example could be made to work if document() was made
stricter and had return type document? (and I believe this may be what
is going to happen in the next draft).  Then a judgment such as

 


  _____  


 <http://www.w3.org/TR/query-semantics/#xq_stat_env_def#xq_stat_env_def>
statEnv |- child::
<http://www.w3.org/TR/query-semantics/#jd_type_filter_on#jd_type_filter_
on> on document
<http://www.w3.org/TR/query-semantics/#jd_type_filter_on#jd_type_filter_
on> : element 

 

 

might help to get everything working.  But this may require a judgment
for element absent of any other decorations, perhaps along the lines of:

 


  _____  


 <http://www.w3.org/TR/query-semantics/#xq_stat_env_def#xq_stat_env_def>
statEnv |- element, *
<http://www.w3.org/TR/query-semantics/#jd_type_filter_on#jd_type_filter_
on> on element
<http://www.w3.org/TR/query-semantics/#jd_type_filter_on#jd_type_filter_
on> : element  

 

 

but I have not fully considered the implications of 'unnamed' elements
at this part of the judgments.  In this example, it seems fairly benign.
The static type of this example would be element*, which I believe is
the desired result.  (Note that the resulting sequence value will only
contain elements, no other node types (not including the children of
these elements).  Is this correct?)

 

Throw in the judgment

 


  _____  


 <http://www.w3.org/TR/query-semantics/#xq_stat_env_def#xq_stat_env_def>
statEnv |- element, QName
<http://www.w3.org/TR/query-semantics/#jd_type_filter_on#jd_type_filter_
on> on element
<http://www.w3.org/TR/query-semantics/#jd_type_filter_on#jd_type_filter_
on> : element QName  

 

for good measure and I believe Example 3 is 'fixed', as well.  Example 3
would now result in a static type of element bar*, which I believe is
desired.  Note that this judgment is slightly 'backwards' when one
considers the other Axis NodeTest judgments.  In this case, we compute
the static type based upon the content on the left-hand side of on,
rather than on the rhs like all of the other similar judgments.

 

GENERAL QUESTION:

 

Consider the normalization:

 


[StepExpr1 "/" StepExpr2]
<http://www.w3.org/TR/query-semantics/#jd_map_expr#jd_map_expr> Expr 


==



fs:distinct-doc-order (


  let $fs:sequence := fs:distinct-doc-order( [StepExpr1]
<http://www.w3.org/TR/query-semantics/#jd_map_expr#jd_map_expr> Expr )
return


  let $fs:last := fn:count($fs:sequence) return


  for $fs:dot at $fs:position in $fs:sequence return


    [StepExpr2]
<http://www.w3.org/TR/query-semantics/#jd_map_expr#jd_map_expr> Expr 


)

 

The static type of StepExpr "/" StepExpr is the static type of
fs:distinct-doc-order, which has the very uninformative static type
node*.  Are we to ignore that fact, and use the static type of the for
expression as the static type of this path expression?  I believe this
is the intent, but there probably should either be some language or some
judgments to this effect.  If we take this literally, then all path
expressions have static type node*.

 

If you made it this far, thanks a bunch!

 

            -Todd