Comment by internet_points

For me, Haskell's image and ideal of purity are what made it difficult when I started out. I tried learning the language by reimplementing a program I'd previously done imperatively, that was (in hindsight) obviously hard to do in a plain pure way, ended up learning about zippers and knot-tying to do something in a less efficient and more confusing way than just using something like STArray because I had this idea from reading about Haskell that this was not only a good way to do things, but would be magically fast because GHC. (It was not.)

These days I'd just do such a task more-or-less imperatively in Haskell, and I would be well guided by the types in doing so. But I also feel like you have to make a few such mistakes if you want to get a good intuition and taste for when it's good do things purely and when imperatively.