• Haskell Playground
• QuickCheck: Automatic testing of Haskell programs
• r/haskell / Solving the expression problem with Object Algebras and Tagless Interpreters

Books

• What I Wish I Knew When Learning Haskell
• Haskell Quick Syntax Reference. A Pocket Guide to the Language, APIs, and Library by Stefania Loredana Nita, Marius Mihailescu - 207 pages
• Retrocomputing with Clash. Haskell for FPGA Hardware Design by GERGŐ ÉRDI
• Practical Haskell. A Real World Guide to Programming by Alejandro Serrano Mena - 595 pages

YouTube

• ruHaskell

Expert commentary

• "I think going in a different direction, coming more from academia, we have a language like Haskell, which we call a functional programming language. That means function in a mathematical sense, not in the sense the other languages are dysfunctional. But a function mathematically has an input and an output and it maps to, you know, with a great deal of mathematical certainty what those are. Haskell is one of those languages that mathematician-type-minded people love; it's sort of a language for geniuses, by geniuses. So you should probably know about it, if only to be able to say, "Well, is this kind of like Haskell?" And if so, then you know you have to hire some really smart people to program in it. Haskell is sort of a modern kind of Lisp in that sense." (Larry Wall)