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

[Roger L Costello <costello@mitre.org>]

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. 

Ditto for SQL.

Now consider the XML family of languages: XSLT, XML Schema, etc. 

XSLT uses (hosts) the XPath language. Here is an excerpt to illustrate:

select="/Bookstore/Book[1]/Title"

The expression /Bookstore/Book[1]/Title is an XPath expression, the other parts are XSLT. So, the format of the "select" is:

select="XPath"

See how XSLT hosts XPath? That is, one language (XSLT) is using another language (XPath).

The XPath language is a pretty large language, with its own grammar. XSLT, independent of XPath, is also a pretty large language, with its own grammar. How to create a single grammar for XSLT that includes XPath? Copy-and-paste the XPath grammar into the XSLT grammar? That is riddled with problems. 

XML Schema hosts two languages: XPath and Regular Expressions (regexes). 

Yikes! How to create a single grammar for that?

The XML language itself hosts two languages: the DTD language and the namespace language.

So, many of the languages in the XML family of languages are multi-language languages.

Other than the XML family of languages, are there languages that are multi-language languages? Or, is this a phenomena unique to the XML family of languages?

/Roger