[nilved]

Is Haskell's `(++) :: [a] -> [a] -> [a]` pure? What if I pass `unsafePerformIO` to it?

For theoretical purposes it might make sense to say pure functions cannot call impure functions, but I think a more practical definition includes functions that are provided as arguments.

[fenomas]

Haven't the faintest. I don't speak Haskell, 's why I asked.

I'd thought pureness was a property of function definitions, irrespective of what they get passed, but living in Javascript land I've never had occasion to be concerned with the distinction. I kind of suspect it's not really something that's useful to talk about.

[nilved]

Not asking you directly, I agree with you and was trying to rephrase the notion.

[tamana]

If you introduce unsafe into a program, it is not pure. You can run a linter or something to check if your program uses unsafe anywhere.

Purity is a spectrum, not a binary, The more impurities you add, the more likely your program will not behave reasonably (as in, not in accordance with the simple logical rules of referential transparency).