What is the purpose * of the ReaderT Monad Transformer?

Question

I am looking at the documentation for Reading Mod and ReaderT monad transformer.

Relevant definitions:

newtype ReaderT k r m a :: forall k. * -> (k -> *) -> k -> *
type Reader r = ReaderT * r Identity

I do not understand what they are doing *in the definitions. In particular, I am trying to get a new monad from ReaderT with IO as the base monad and class restriction by value r.

I'm not sure why there is a fourth input for ReaderT ( k) and what Reader does with this value when it puts * at this position.

Answer 1

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