Additionally, at least in (GHC) Haskell, you can recover most of the flexibility typical of dynamic languages via use of undefined values, partial functions, th

Additionally, at least in (GHC) Haskell, you can recover most of the flexibility typical of dynamic languages via use of undefined values, partial functions, the PartialTypeSignatures extension, and the defer-type-errors flag. Personally, I've found these tools to be very useful when prototyping, exploring/experimenting in a REPL, or beginning a refactor, and the process feels very akin to development without unit tests (or tests turned off/ignored/whatever), especially in the progression of incrementally adding back compile-time-checks and watching my mistakes surface.