Re: Are multi-language languages unique to the XML family of languages?

On Sun, 6 Mar. 2022, 10:02 Roger L Costello, <[email protected]> wrote:

> Hi Folks,
>
> Consider the C language. It is one language. It doesn't use (host) other
> languages. Consequently, it is relatively straightforward to create a
> single grammar for the C language. Once the grammar is created, a robust
> parser can be created.
>
> Ditto for every other programming language.
>

No.

It is common to embed other query languages in C, Java, etc:  SQL, Regex,
Xpath, URLs, even shell commands. Common practise for 40+ years.  And
almost every language has conventions for formatted strings with variable
substitutions.

In fact nowadays languages --as used-- are *less* standard than ever: if
you use C or Java or most general purpose languages, you may well be using
annotations or other extensions and preprocessors.  And you may be
generating JavaScript, CSS, JSON, HTML and so on, but be developing in
Typescript, Less, ExtJSON, templated HTML. Angular is a good example of
this.

In fact, all the little standard languages that prided themselves on
idealistic "simplicity" almost immediately degenerated into, in effect,
mutually incompatible dialects as different pre-processors, mostly doing
the same things, proliferated.  Why don't we just call a spade a spade:
omitting support for the things needed for basic software engineering does
not make a little language "minimalist", it makes it "fatuous". For all our
YAintGNI bravado, it always turned out YAreGNI.

The exception? XML, where there was less need to invent annotations
(attributes or PIs), server side includes (general entities), simple
macros, templates (attributes or PIs), let alone comments (comments).  And
where many of the more complex preprocessings could be done in ubituitous
XSLT.

In traditional SGML terminology, I think Roger is saying that because XML
schema languages have abandoned any idea of "NOTATION"  for attribute
values or data content, there is no standard way either in markup or in
schemas to label strings according to their little language. So there is no
standard "seamless" way for an IDE to be able to colour that text or
specially validate it.  (This is the same issue as not knowing if a qname
in content is content, b.t.w.)

So Roger cannot create a robust parser without some non-standard
information. Not just to parse some embedded notation, but even to know
which data is of that notation.

(IDEs solve this in 2 ways: first, standard libraries such as string
formatting libraries are treated as special cases and built in; second,
annotation processors allow extra information that plugins can use. )

So to Roger's questions: no, embedding little languages is ubiquitous; yes,
XML does not have good support for a parser to know what embedded notations
are used and where; and I think this was an own-goal that unnecessarily
reduced the utility of XML in favour of the things it is not particularly
good for: sending labelled simple strings around.

<Digging up the dead>
I recall raising this issue of embedded notations in data at the W3C XML
Schema discussions. I recall two answers: "whatever the answer is, it isn't
notations"  and "you cannot do anything without types" (I.e. if you have
data types and lists/unions etc that exhausted the useful things an XML
document needed: i.e. an XML document was a serialization of a database or
object only.)   ...As I understand it, most XSLT is still done without
types (if we ignore the extra @as'es that XSLT2+ demands we put in) which
puts paid to the second comment?

I do have some sympathy for the view that syntax is not a schema issue
(though wouldn't that kinda undercuts all lexical datatyping? ...
nevermind) and so notation identification is some other kind of
declaration: e.g.
   <query  XML:NOTATION="SQL">SELECT x FROM y</query>
or
    <query><? XML:NOTATION name="SQL"?>SELECT x FROM y</query>
or
     <thing  query="&lt;?XML:NOTATION name='SQL'?>SELECT x FROM y" />
or
      <?XML:NOTATION path="query" name="SQL" ?>
       ...
       <query>SELECT x FROM y</query>

One thing XML DTDs ditched was the ability to declare that an attribute
value is to be treated not as data but as a PI. XML Schemas followed this.
So there was no standard way for a parser to know this was some special
information (let alone resolve the target.)  So XML is in this situation
where the decision not to treat PIs and NOTATION as first class citizens
has reduced its applicability, or rather, provides no help for parsing
embedded little languages using standard declarations.

I would have liked something like this, where the first token is some
simple target like other PIs:
 <!ATTLIST thing query PI #REQUIRED>
...
<thing query="sql SELECT x FROM y" />
Just enough to know the value is special, and to hint what it is. But many
other approaches are fine.

Regards
Rick

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