Covariant

Covariant[F] provides implicit evidence that F[+_] is a covariant endofunctor in the category of Scala objects.

Covariant instances of type F[A] "produce" values of type A in some sense. In some cases, such as with a List[A], this means that they contain values of type A, in which case we can simply access the elements of the collection. In other cases it means that output values of type A which may not already exists, such as with a Function0[A] that produces A values when invoked.

Common examples of covariant instances in ZIO includes effects with respect to their error and value types, sinks with respect to their error and output types, and queues and references with respect to their error and output types.

Covariant instances support a map operation which allows transforming the output type given a function from the old output type to the new output type. For example, if we have a List[String] and a function String => Int that returns the length of a string, then we can construct a List[Int] with the length of each string.