Refactoring Haskell code has been the best refactoring experience I've ever had, and I've worked with "a lot" of imperative languages. IMO the biggest boost in

Refactoring Haskell code has been the best refactoring experience I've ever had, and I've worked with "a lot" of imperative languages. IMO the biggest boost in maintainability is by adding a (typed) compilation step - after that the stronger the type system, the better.