In my experience at IMVU, surprisingly not very difficult. It's easier to learn Haskell in a team environment than on your own because your peers can help you

In my experience at IMVU, surprisingly not very difficult. It's easier to learn Haskell in a team environment than on your own because your peers can help you connect the dots and accomplish your task at hand. It did seem like less experienced people picked it up faster, perhaps because they had less to unlearn. The hardest things were learning to parse GHC's error messages and learning all of the names of the functions to transform between various data types.