Since when does Functional === Computer Science?
Even more, I wouldn't use immutable functional programming for writing an OS. An OS is all about mutating state that keeps track of resources.
For that matter, I wouldn't use functional programming in any situation where the nature of the problem cried out for mutating state.
I wouldn't use garbage-collected code to write a memory manager. I wouldn't use it to write crypto code (leaving old crypto data hanging around in memory is a major no-no).
But we get these loud zealots who tell us that FP is The One Right Way, and that we're stupid and/or ignorant if we don't convert at once. I suspect that a lot of people with experience just tune them out, because it's so obvious that what they're saying is unrealistic. But if they were a bit more aware of the variety of the real world of programming, and knew where FP worked and where it really didn't fit, they might get more of a hearing.
It's nice to see these two words used to describe the benefits of functional programming. One often hears "expressivity" and it's less clear to me what that means, precisely.
