pure-fun | Javascript pure functions/helpers with 0 dependencies | Functional Programming library

A function is pure if its result (the return value) only depends on the inputs you give to it.

A language is purely functional if all its functions are pure¹.

Therefore it's clear that "utilities" like getchar, which are fairly common functions in many ordinary, non-functional languages, pose a problem in functional languages, because they take no input², and still they give different outputs everytime.

It looks like a functional language needs to give up on purity at least for doing I/O, doesn't it?

Not quite. If a language wants to be purely functional, it can't ever break function purity, not even for doing I/O. Still it needs to be useful. You do need to get things done with it, or, as you say, you need

something that can actually do something

If that's the case, how can a purely functional language, like Haskell, stay pure and yet provide you with utilities to interact with keyboard, terminal, and so on? Or, in other words, how can purely functional languages provide you with entities that have the same "read the user input" role of those impure functions of ordinary languages?

The trick is that those functions are secretly (and platonically) taking one more argument, in addition to the 0 or more arguments they'd have in other languages, and spitting out an additional return value: these two guys are the "real world" before and after the "action" that function performs. It's a bit like saying that the signatures of getchar and putchar are not