Are side-effects possible in pure functional programming

Question

I have been trying to wrap my head around functional programming for a while now? I have looked up lambda calculus, LISP, OCML, F# and even combinatorial logic but the main

Answer 1

Interacting with a user and communicating with a remote service do require some sort of non-functional part to your software.

Many "functional languages", (like most Lisps) are not purely functional. They still let you do things with side-effects, though side-effecty things are "discouraged" in most contexts.

Haskell is "purely functional" but still lets you do non-functional things via the IO monad. The basic idea is that your purely functional program emits a lazy data structure which is evaluated by a non-functional program (which you don't write, it's part of the environment). One could argue that this data structure itself is an imperative program. So you're sort of doing imperative meta-programming in a functional language.

Ignoring which approach is "better", the goal in both cases is to create a separation between the functional and non-functional parts of your programs, and to limit the size of the non-functional parts as much as possible. The functional parts tend to be more reusable, testable, and easier to reason about.