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I personally didn't understand FP at all until I played around with Erlang (specifically, the "Learn You Some Erlang" book). I reckon Elixir would be similarly enlightening (if not more; I feel like Elixir's macros are easier to wield than most Erlang equivalents, for example).

The friendly, consistent syntax and the imperative-ish constructs in some places can work as a great stepping stone into the rest of the functional/declarative programming world.