Why aren't there many discussions about co- and contra-variance in Haskell (as opposed to Scala or C#)?

Question

I know what covariance and contravariance of types are. My question is why haven\'t I encountered discussion of these concepts yet in my study of Haskell (as opposed to, say

Answer 1

There are no "sub-types" in Haskell, so covariance and contravariance don't make any sense.

In Scala, you have e.g. Option[+A] with the subclasses Some[+A] and None. You have to provide the covariance annotations + to say that an Option[Foo] is an Option[Bar] if Foo extends Bar. Because of the presence of sub-types, this is necessary.

In Haskell, there are no sub-types. The equivalent of Option in Haskell, called Maybe, has this definition:

data Maybe a = Nothing | Just a

The type variable a can only ever be one type, so no further information about it is necessary.