Most of my Erlang programming is side effect free. I think I probably write very unusual Erlang programs that look a lot like Haskell ones. Now and then, I do write side effecting code. For example, when I use the random number generation libraries that comes with Erlang, it has a side effecting interface. It’s very tempting when you are building something on top of the library with a stateful interface to build code on top of that that also has a stateful interface.

That’s what I did the first time I tried to use it. That has caused me so much trouble. I think every single bug that I spent hours trying to track down has been caused by a side effect. In a way, I’ve been programming Haskell for so many years, that I’d forgotten just how devastating side effects are and just how difficult they make debugging. I’ve learnt that lesson again and nowadays, if I want to use a stateful library, I usually build a side effect free interface on top of it, so that I can then use it safely in the rest of my code.

from my interview with John Hughes at Erlang Factory