By “pure”, I mean lacking unaccounted side effects (including crashing and non-termination).

I take a rather strong view, as you can tell, of what it means to “support” pure functional programming. That is, I think it is insufficient to show that your code lacks appeals to falsehood and rampant non-termination. Because your code is linked with other code from other sources, it must be shown that appeals to falsehood and rampant non-termination are in fact _impossible_.

(By “rampant non-termination” and “appeals to falsehood”, I mean the use of circular reasoning. This is distinct from structural recursion and coinduction, which can be used to do REPLs and servers.)

So yes, Virginia, there is indeed a Pure Functional Programming. But you can't do it in Haskell.