[fenomas]

That's what I thought. It boils down to the question of whether:

    f => f()

is a pure function? If so, then so is the author's example. If not, then I don't see how any JS function that takes an argument (and evaluates it) can be pure. Under that definition, I'm not sure it makes sense to talk about.

[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).

[masklinn]

> If not, then I don't see how any JS function that takes an argument (and evaluates it) can be pure.

A JS function could take an argument, use it but not evaluate it (or not evaluate it in all cases). `typeof` doesn't evaluate its argument for instance.

[fenomas]

Certainly it could. I don't know what that has to do with my comment though.