I was in a similar boat to yourself last year, and teaching myself Elm (elm-lang.org) was a refreshing change because I hadn't done functional programming before. It's less hardcore than OCaml or Haskell but has most of the essential features of these languages incorporated in a much better designed and coherent way.
Although I don't find the language very practical for everyday usage, the concepts (immutable objects, reactive data flow, pure functions, partial application, etc.) I found useful and applicable to my work in other mainstream languages. Many are incorporating functional techniques, design patterns and features into them.
The Elm Guide (guide.elm-lang.org) was a great introduction to the language for me.