Haskell is very much the deep end of learning functional programming - it's at an extreme end of the spectrum in purity and type system. The language I recomme

Haskell is very much the deep end of learning functional programming - it's at an extreme end of the spectrum in purity and type system. The language I recommend as a first step to most people is Elixir (or Erlang). It's fairly pure, data is immutable, relies on recursion at the lowest level etc. Good code in Elixir is structure in a similar to good code in a lot of other functional languages (and so teaches good habits), but being dynamically typed is avoids that immediate pain of learning about how to keep the compiler happy.