[bgamari]

> The iterative version in a number of languages is 10 lines, but the functional version needs some way of drilling down into an array in a mutable way (a monad) in order to make it match the iterative in performance

Sure, but IMHO the monadic code necessary to implement something like Arnoldi is neither harder to write nor uglier than the same implementation in an imperative language.

If anything, this highlights a strength of the language: we can capture an imperative idea in a functional setting, and even have the ability to nicely isolate the mutability from the rest of our functional program (with `ST`).