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

The only completely pure functional language I know of is the template system in C++. Haskell takes second place by making the imperative portions of the program explicit.

In Haskell the program has mutable state, but functions (almost always) don't. You keep like 99% percent of program pure, and only the portion that interacts with the outside world is impure. Therefore when you are testing a function, you know there are no side effects. Pure core, with an impure shell.