I've been trying to crack Haskell for a while, but didn't find it approachable at first. Somehow Elm and building simple web apps made grasping FP easier.
So now that I've learnt a bit of Elm, I find I can grasp Haskell more and spend a bit playing with. The best part is, after reading this [0] I'm finally grasping Monads.
I learnt more about Haskell in ten days of using Elm than I would have in another ten days of studying Haskell, personally.
I think Elm makes a better introduction to FP concepts because there's much less you have to absorb before you reach the point where you can start practicing by doing useful work. Obviously part of that is the fact that Elm removes or hides certain things Haskell has, but an even bigger reason is that you can just say "...and then 'main' returns the HTML element or Html.program that actually gets displayed" and not have to go down the road of IO actions, functors, etc. You can stop at that point and start making working useful applications while getting comfortable with immutability, purity, the type system, and control flow and iteration under those constraints.
Learning Haskell first, you don't have that opportunity to stop and start practicing. You need to move on an understand at least IO actions, functors, applicatives, typeclasses, and other higher-level concepts before you can construct even a simple practice project. Dreaming up a coherent program structure/flow in this weird new immutable and pure world seems hard enough to a beginner without also having to understand how applicative functors fit into the equation. Having that opportunity to stop and stretch your legs by actually doing a project is a major help to a lot of people, that's what makes all the difference. And then 95% of what you've learned transfers directly into Haskell.
I would recommend having a look at Purescript too. One of the very nice things about Purescript is that it generates very readable Javascript. One of the things I was always wondering about is how exactly tail call optimisation works. Looking at the output of Purescript for about 10 seconds answered any questions I had.
Also, though it has been posted here before (and I don't think it has been worked on for a while), I recommend playing with the Monad Challenges[0]. Well, specifically, just do set one (random numbers). You can easily write your own rand function that returns the seed value as a "random" number and then increments the seed. This will generate successive integers (1,2,3,4...). It makes it very easy to test. Then once you've done set one, go back and write map, apply, etc for Gen. One other nice thing you can do is to make a union type/ADT for your random number (i.e., (Int, Seed) ) and then try to see if it is a functor and applicative (also try to understand why or why not). Finally, you can figure out why Gen is structured the way it is.
I've played with that kata over and over and over again. It is simply beautiful.
So now that I've learnt a bit of Elm, I find I can grasp Haskell more and spend a bit playing with. The best part is, after reading this [0] I'm finally grasping Monads.
[0] http://adit.io/posts/2013-04-17-functors,_applicatives,_and_...