He's complaining about mutable state, essentially. The function `f` captures reference to `i` but does so transparently. One might expect that `f` would be free

He's complaining about mutable state, essentially. The function `f` captures reference to `i` but does so transparently. One might expect that `f` would be free of side effects, but since `i` can be re-bound whenever this is not the case. Essentially it comes down to the semantics of `=` whether it's declaration or assignment. Swift appears to choose it to be assignment. If I remember right you can declare things constant which would effect declaration only (sort of) but clearly there's an expectation that this happens all the time.