Earlier quoted context omitted.
I have ~4 years experience writing OCaml, and ~1 year experience writing Standard ML. I also once did contracting for a Play app in Scala for 3 months. A month ago, I started looking into Haskell for the first time for another side project and was immediately able to pick it up due to how similar it is to OCaml and Standard ML. In particular, the type syntax and function declarations are similar; understanding monads…
If you already know SML or OCaml, you'll pick up Haskell easier, but if you don't know them and want to learn Haskell, you're not taking a shortcut by learning SML or OCaml first.
Its whatever works for every person, but I'll throw in my own experiences and say, that by taking a detour into Ocaml I was actually greatly aided in understanding Haskell. And that having spent a great deal of time with a number of Lisps. I had grabbed a copy of RWH, dug into it, been a little defeated at first, took a studious detour through RWOcaml and have returned to Haskell with a clearer view. Again, to each their own, my own experience was actually the opposite of your statement.