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Well, avoiding mutation can be helpful but it's not a rule to be followed blindly either. A mutable local variable never hurt anyone, so long as it doesn't leak out of the function.

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Even in a multiple-thread scenario?

If it doesn't leak out of the function (though a closure or a pointer), how can another thread see it? It's only on one stack.

As an aside, one of the really interesting ideas that's bubbling up from Rust is that it's not mutable state that's a problem, it's shared mutable state, and that you can make it safe by removing the sharing, rather than removing the mutability:

http://smallcultfollowing.com/babysteps/blog/2014/05/13/focu...

I have no idea if this is true, but it's going to be really interesting to see that idea tried.

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