A function which returns the same output (and has no side effects) if given the same input is a "pure function". Things like network requests, or reading from

A function which returns the same output (and has no side effects) if given the same input is a "pure function". Things like network requests, or reading from files, or keeping track of state like a counter, are effectful. So, yes, a function which does these things couldn't be pure. It's still nice to be able to have a pure function over an impure function. (Sortof the same "nice" as in "it's nice to not use global variables for everything in your code"). An impure function might be more difficult to think about than a pure one. In languages like Haskell, all the functions are pure. But useful programs are effectful. There are different ways Haskell can still have useful programs while only allowing pure functions. (The most common way this gets done is using monads). -- One trick is "algebraic effects", this blogpost is an accessible intro: https://overreacted.io/algebraic-effects-for-the-rest-of-us/ https://overreacted.io/algebraic-effects-for-the-rest-of-us/ Functional languages themselves are about a bit more than pure functions. It's about being able to pass functions around.. languages like JavaScript or Python have functions like map/filter/flatMap. Functions which take functions as parameters. They're a neat way of writing code. The JSON program JQ's language is comparable to Nix. -- You could write your JSON-processing programs with Python (or whatever), or you can use some JQ expressions to do the processing.