I spent a lot of time learning Haskell early in my career. I doubt I'll ever write an actual program in Haskell. I probably won't write so much as a single line of it that will ever see production. (Thank goodness. Haskell is far from perfect, despite its many zealots.) But learning Haskell has changed the way I write code in any language. It's helped me to write better, more testable, more readable, more maintainable, more bug-free Javascript. I wrote a lot of Clojure for a while a few years ago and being familiar with Haskell dropped the learning curve of Clojure down to almost nothing. I personally adore Lisps more than Haskell, but Haskell unlocked the world of functional programming for me. It also showed me the true power strong typing has at a time when the only strong typing I was aware of was the dismal type system of Java. (Like late 90s Java at that!)