[xquery-talk] How do different XQuery products implement modules?

[Per Bothner]

Answers for Qexo:

[I wrote the code a number of years ago, so in some cases my
memory might be off, so I am looking at teh code to refresh
my memory.]

Michael Kay wrote:

> 1. Is the module URI just an arbitrary character string, or does it have any
> semantics?

It can be be arbitrary string, but it is "mangled" to the name of a
"package" in the Java class hierarchy, similar to the JAXB mangling.

>    1a. In particular, is it the location of a resource on the web?

No.

> 2. Do you allow several different modules in the same query to have the same
> module URI?

Yes.

>    2a. if so, can different "import module" declarations import different
> subsets of these?

Possibly, but it isn't supported or recommended.

> 3. Can users import a module without specifying a location URI?

Yes, but the module has to either have been previously compiled
to a Java class (in the current class path), or the imported
module must have been "registered" with the module manager - which
happens when compiling multiple modules on the same "compilation line".

 > If so, how is the module located?

It mangles the namespace to a package name.  It then looks for a class
named $ModuleMap$ in that packages.  This class contains a list of all
modules whose namespace mangle to this package name.  The compiler
loads the $ModuleMap$ class, and selects the modules whose namespace
URIs match.

When a module is compiled, the compiler creates a class in the package
corresponding to the module's URI.  The name of the class is a mangling
of the last component of the source file name, within the package
created by mangling the namespace URI.  The compiler then updates the
$ModuleMap$ to include the compiled module(s).  If $ModuleMap$ already
exists, then the class is first is loaded so the old contents can be
merged into the $ModuleMap$ that gets written out.

> 4. If import module specifies a location URI, is it used, and if so how?

A source file is looked for.  It also mangles the source file name
and the namespace URI to yield a class name, and looks for that as well. 
  It will work if it finds either.  If both are found, it will compre
time-stamps.

> 5. If import module specifies more than one location URI, how are they used?

Each location URI must map to a module; all are loaded.

>    5b. If multiple location URIs are allowed, what happens if
>    import module specifies the same location URI more than once?

Hm.  That should be an error, but I don't think I test that,

> 6. What happens when two modules A and B both import module URI M
> 
>    6a. specifying the same location URI

Same module.

>    6b. specifying different location URIs

Not sure - it should be an error.

>    6c. without specifying a location URI

Same module, found using mangled class - see 3,

> 7. Is it possible (by means of multiple imports) to create two different
> "instances" of the same module in a query, with distinct values of global
> variables, or do all imports get the same instance with a common set of
> variables?

The latter.  By default a module variables are compiled to static fields
of the module class, so they're inherently shared.  There is a compiler
option to compile to instance variables.  In that case a single "module
instance" will be allocated, global to the current "module context".
-- 
	--Per Bothner
per at bothner.com   http://per.bothner.com/