"Of course, it should be noted that BEAM isn't entirely immutable." Mutability is relative to the layer you're looking at. BEAM is, of course, completely mutab

"Of course, it should be noted that BEAM isn't entirely immutable." Mutability is relative to the layer you're looking at. BEAM is, of course, completely mutable from top to bottom because it is constantly mutating RAM, except, of course, that's not really a helpful way of looking at it, because at the layer of abstraction you program at values are immutable. Mutatable programs can be written in terms of immutable abstractions with a well-known at-most O(n log n) penalty, and immutable programs can be written on a mutable substrate by being very careful never to visibly violate the abstraction of immutability, which is good since there is (effectively for the purposes of this conversation) no such thing as "immutable RAM". (That is, yes, I'm aware of WORM as a category of storage, but it's not what this conversation is about.)