A Haskell tutorial?! it'll be good to have. Given how brief the lessons here are, not much can be covered. Hopefully at the end of the lessons, instead of just completing it with certificate, they include some pointers on where to go for next steps in furthering knowledge.
Functional programming in Haskell is a great missing piece in Sololearn. Given how fp concepts being so different from the usual imperative programming, the ensuing collaborative discussion would enormously bring out lambda calculus notions out into the light. I tried to create Haskell basics questions in my Android smartphone but am currently unable to. I am no Haskell expert, but I know the basics and my noob thoughts would resonate well to beginners. And sometimes, even my mistakes could be fertile ground for not fearing Haskell.