[chuckadams]

You need them to express a type constraint that’s generic for all monads. I might have gone a bit too far saying monads would be impossible without HKTs, but a single Monad type class would be, and you lose a lot of the power of monads if you don’t have that.

[fire_lake]

You can do monads without HKT but it is much more manual. You need to compose monad transformers yourself, for example.