One thing that was a personal shocker for me to hear, from not just anyone, but Matthias Felleisen himself - "My research group has investigated the topic for almost 3 decades, and we came to the conclusion that Hindley-Millner doesn't really work". I was really baffled. That was at ClojureWest 2016. Here's the talk itself https://www.youtube.com/watch?v=XTl7Jn_kmio. That talk is not about Clojure - Felleisen doesn't even use Clojure. That happened before Clojure.spec was announced. I couldn't really figure out what exactly he was talking about. Not until later, when I tried Spec.