I agree that sometimes it helps to have the type system in non-trivial situations, but in OCaml, you could do this imperatively, keep the state change isolated

I agree that sometimes it helps to have the type system in non-trivial situations, but in OCaml, you could do this imperatively, keep the state change isolated to a specific function (or module), and still give it a pure functional interface. The type system would do the same checks, even. (It sounds like that's what happens with Haskell's Data.Array.Diff.) At the same time, many programs in the real world get by without sophisticated type systems. They're a great tool, but they're also probably vastly more useful in some problem domains than others. Same with OOP, constraints and backtracking, relational databases, etc. (That said, I would love to use an ML-like language as small & clean as Lua. Inferred, garbage-collected, recursive tagged-union types rule.)