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Uche Ogbuji wrote:
> Static typing was there [in Eiffel], for the same reason
> that static typing emerged in compilers in the first place:
> to take advantage of physical layer facilities provided
> by the hardware.
> [...]
> Static typing is a necessary evil for systems at that
> lower level: for performance reasons. Being that XML is
> at a higher level of processing, it should not have to be
> bound to this evil. At least not explicitly.
> [...]
> Because Meyer was constructing a compiler: he had to deal
> directly with the physical layer. I think one of the
> strengths of XML is that technologies at the XML layer
> should not have to.
But the XQuery type system operates at a higher level too.
There's more to it than detecting when a programmer tries
to multiply an xs:anyUri by an xs:integer. The type system
can also answer questions like "If I evaluate this query
against a valid PurchaseOrder, will I get back a valid
PackagingList?"
There *is* a tremendous utility in XML type systems; without
DTDs or their moral equivalent we might as well be trying
to process Word documents.
Whether or not XQuery's extension of a type system for data to
a type system for programs will be useful in practice is IMO still
an open question. I think Uche is _probably_ right, that it won't
be much of a benefit, but I'm trying to keep an open mind about it;
there's a lot more there than just the low-level physical-layer
stuff.
--Joe English
jenglish@flightlab.com
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