Possibly not too high. Try using map, filter, and reduce on some example problems and you'll pick it up fairly quickly. Project Euler makes for decent fodder. When you've successfully managed to apply them then try rewriting the functions (delete the implementation you are using - such as the library above - and rewrite it from scratch).
I wouldn't recommend Project Euler - it's math-based and that would be a roundabout way of learning anything else but math.
It's a pretty fast paced intro to a lot of functional programming concepts. You can read a guide like learnyouahaskell.com, which will take you on a gentler ride.
Thanks I tried haskell a few ties but never got the real life application to that.