Немного вне темы, но можно поднять трубку VisualVM и JConsole к запущенным приложениям и видеть полезную статистику.
There's a good description of this in the ocaml tutorial. What's happened is you have shadowed a type definition with a new definition:
type nfa = int
let f (x: nfa) = x
type nfa = int
let g (x: nfa) = x
Restarting the top-level will clear out the old definitions.