Re: [xml-dev] which class of programming languages, XML Schema language

[Marcus Reichardt <u123724@gmail.com>]

It's not entirely clear CSS isn't Turing-complete yet though, or bound to become Turing-complete soon.

Not talking about that old "proof" where the user has to perform clicks to function as tape, but rather about newer CSS features such as var() in combination with grid and other constraint-based layouts etc. which I'd not be surprised to hear having become accidentally Turing-complete considering the grab bag CSS has amounted to.

Apart from that, how about XSD is a regular tree grammar? Though with unconstrained XPath assertions or XPath 2+ existential quantifiers etc. XSD might eventually end up becoming Turing-complete, too.

FWIW, it has long been established that XSD is undecidable ie. the problem of answering whether an arbitrary XSD can have a valid instance isn't decidable in general due to interaction of key constraints with minOccurs/maxOccurs as I recall, even without XPath assertions.

Best,
Marcus Reichardt
sgml.io


> Am 11.03.2022 um 13:29 schrieb Pete Cordell <pete++xmldev@codalogic.com>:
> 
> On 11/03/2022 08:48, Michael Kay wrote:
>> Well, I would say that XSD is indeed a "formal computer language", but not a "programming language"; I don't think you can describe something as a programming language unless it is Turing-complete.
>> I don't know of any universally accepted categorisation scheme for formal computer languages, and without such a scheme you can't say where a particular language fits; but it's certainly reasonable to describe XSD as a constraint specification language or as a data definition language (if indeed those two categories are distinct).
> 
> A related perma-topic is "Is HTML a programming language?"
> 
> I'm in the "No, because it is not Turing-complete" camp (for example, it has no conditionals) but it does instruct a computer to do things and so others say it is.
> 
> I'd say if "programming language" was a spectrum, HTML would be more of a programming language than XSD.
> 
> To add more confusion, for most programming languages such as C++ and Java, it's very specific what the language tells the computer to do.
> 
> With XSD, the XSD can be used in many ways by a computer.  Does that suggest it is "something else"?
> 
> Pete.
> -- 
> ---------------------------------------------------------------------
> Pete Cordell
> Codalogic Ltd
> Read & write XML in C++, http://www.xml2cpp.com
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