Question

Several questions about functional programming languages have got me thinking about whether XSLT is a functional programming language. If not, what features are missing? Has XSLT 2.0 shortened or closed the gap?

Answer 1

XSLT is declarative as opposed to stateful.

Although XSLT is based on functional programming ideas, it is not a full functional programming language, it lacks the ability to treat functions as a first class data type. It has elements like lazy evaluation to reduce unneeded evaluation and also the absence of explicit loops.

Like a functional language though, I would think that it can be nicely parallelized with automatic safe multi threading across several processors.

From Wikipedia on XSLT:

As a language, XSLT is influenced by functional languages, and by text-based pattern matching languages like SNOBOL and awk. Its most direct predecessor was DSSSL, a language that performed the same function for SGML that XSLT performs for XML. XSLT can also be considered as a template processor.

Here is a great site on using XSLT as a functional language with the help of FXSL. FXSL is a library that implements support for higher-order functions.

Because of FXSL I don’t think that XSLT has a need to be fully functional itself. Perhaps FXSL will be included as a W3C standard in the future, but I have no evidence of this.