[StreamBright]

Exactly. You can use FP ideas in pretty much any language (and most people do who like reliability). People who do not see value in immutability do all sorts of tricks to avoid the pitfalls of sharing mutable state. One example is in Java design patterns when they recommend creating a copy of the object you are handling. I can't remember the exact name of this pattern, but it is kind of funny.

[jierenchen]

Or the visitor pattern, which is.... the map function.

One thing I will note from recent experience: programming for a long time with immutability cripples your ability to think about mutable things. I was writing up a streaming merge sort this week and it was a brutal nightmare because of all the state I had to deal with. Seems like a call to action to deal with a bit of mutable state every now and then. Everything is too pure these days. We're programmers, not mathematicians dammit.