OOP contains in-place mutable state because mutating state is
extremely important in basically all computer systems.
FP has become fashionable in recent years and now it's gone to people's heads. But some difficult facts about computer science remain:
- CPUs are much faster at reading and writing to recently used locations. Mutating in place is fast compared to constantly copying things.
- Many of the most efficient data structures and algorithms require mutable state. There are entire areas of computer science where you cannot implement them efficiently without mutable state, like hash tables.
- Constantly copying immutable objects places enormous load on the GC. This is getting better with time (there are open source ultra-low-pause GCs now), but, Rich Hickey just sort of blows this off with a comment that the "GC will clean up the no longer referenced past". Sure it will: at a price.
- He repeats the whole canard about FP being the only way to exploit parallel programming. People have been claiming this for decades and it's not true. The biggest parallel computations on the planet not so long ago were MapReduce jobs at Google: written in C++. Yes, the over-arching framework was (vaguely) FP inspired. But no actual FP languages appeared anywhere, the content of the maps and reductions were fully imperative and the MapReduce API was itself OO C++!
Also note that Java has a rather advanced parallel streams and fork/join framework for doing data parallel computation. I never once saw it used in many years of reading Java. SIMD is a much more useful technique but nearly all SIMD code is written in C++, or the result of Java auto-vectorisation. FP programming, again, doesn't have any edge here.