Still a language more academic than commercial (it doesn’t support JVM and Android, unlike Scala which is both O-O and FP, also on JVM), Haskell is the language to learn together with the Advanced Math “Category Theory“.
One FP developer says:
“My heart (Love) is with Haskell, my brain (practical) with Scala.”
This limitation will change when more companies adopt Haskell (eg. Facebook uses Haskell in AI screening fake news, etc)
Excellent eBook (for beginners):