Kind occurrence check deferred to the object definition.

[Heimdell]

The next code compiles fine:

module hello.HelloWorld where

type Omega s = s s

data Cons a b = Cons a b

type Stream a = Omega (Cons a)

--infinite :: a -> Stream a
--infinite x = Cons x (infinite x)

data Const a b = Const { getConst :: a }

although, Omega type should clearly produce a "cyclic kind" error.

If we uncomment the infinite function, then compiler complains on Stream type.

GHC will yell on the Omega itself, which is the best behaviour for me.

[Ingo60]

Yes, currently the compiler only checks that the type definitions don't construct cycles. As long as you don't use them, you can write utter nonsense on the RHS.

[Ingo60]

To solve this, a more advanced kind inference is needed.