[microtonal]

From personal experience: I didn't make much progress in Haskell until I stopped using Scala. The problem is that Scala allows you to mix and match different paradigms and if you come from a mostly-imperative/OO background, you tend to use Scala as an OO language with some functional constructs.

To learn to program purely functional, it's best to jump into Haskell cold-turkey, since you will have to learn to think in FP.

Learning Haskell, optimization in a lazy world was the most difficult task. Often, I still have problems predicting how efficient particular code will be. The complexity of monads is somewhat overstated, though it doesn't help that some tutorials make something big and esoteric out of it. It is nothing more than a type class, that specifies how to combine computations that result in some 'boxed value'.