Comment by robert_tweed

To quote Haskeller Michael Snoyman (linked elsethread):

  Please don't insist on calling your bastard child of a language "functional."

It's a declarative language. It's functional language if you squint at it, sort of. Maybe. It certainly doesn't fit with the most broadly acceptable (stricter) definitions of what a functional language is.

The biggest problem, with XSLT 1 at least (and that's primarily what I'm talking about because that's what exists in the wild), is that the output of a template function is a string, whereas the input is a node tree. So to get generalised recursion or perform function composition over the input data, you need to do some evil tricks.

To give the impression of Turing completeness, you need to first parse the input stream by splitting it up into strings and then call functions that perform string processing, which means you can no longer use xpath, or any of the things that really make XSLT. At that point you're not really writing XSLT anymore, you're doing simple recursive string processing using XSLT as a horrendously overweight and improperly-equipped wrapper around your native string libraries.

Incidentally, I don't accept the linked article as a proof of Turing completeness, since it only implements some trivial programs using Turing machines. That's not the same thing as proving equivalence with a universal Turing machine. However, others seem to have done it, presumably using tricks such as string processing, or ignoring the input altogether.

You could for instance, recreate Conway's Game of Life using pure XSLT, but it wouldn't contain any actual XML transformation code and all the state would exist in the running XSLT program, not the XML document. It would also blow the stack as most XSLT engines don't support tail recursion and there's no other way to loop indefinitely.

What you certainly can't do is have one function that outputs a set of XML nodes and another function that has a for-each that consumes that set, or have the ability to run apply-templates over it. That is nothing like functional programming.