It looks like type-checking is becoming the thing to do for Python and Ruby. Prototype quickly, grow your product or service, and then start adding type-checking to ensure higher quality code and keep a competitive edge in terms of bug fixing and delivering more features.

• Stripe created Sorbet for static type checking of their Ruby code.
• Dropbox type-checked 4 million lines of Python code.

When I was coding in Common Lisp about a decade ago, what I really enjoyed about the SBCL compiler was that it had optional types.

So you could iterate quickly with prototypes and later on add types. What I especially enjoyed though was that the type-checking was used by the Common Lisp compiler to create more optimal code.

Nice to see other languages catching on to a great idea.