[kyllo]

Naturally it does, because a good typechecker is like an automatic test suite that guides you through refactoring. It means you need fewer unit tests, which means less test code to maintain.

I am working on a Rails project and a Haskell project simultaneously; the Haskell project requires far fewer tests because the compiler can catch so many potential bugs. Then when you add ghci (Haskell REPL) it becomes almost as flexible and exploratory as a dynamic language. Some Haskellers even like to say that TDD stands for Type-Driven Development. Combine that with REPL-driven development, add in property-based generative testing with Quickcheck, and you have a lot of very powerful tools to help you build the thing right.