And I dont understand why the IDE says a Nothing i...

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[carlos cdmp]

And I dont understand why the IDE says a Nothing is required

[diesieben07]

* is equivalent to in Nothing when writing values.
* means you don't know what the type is, so you have no way of putting any value in, since there is no way to prove that it matches the actual type (since you don't know what the actual type is).

[streetsofboston]

When you use the wildcard as a type-parameter value, you can basically only really use the methods and properties of that interface of class that don’t need that type-parameter at all. E.g MutableList<*> will be problematic when trying to call add (and even call get, since it’ll return an Any?), but calling size or toString() is fine….

[carlos cdmp]

Then how can i write

val middleWareMap: Map<KClass<*>,Middleware<AppState, *any class extending action*,  *any class extending action*>>

[streetsofboston]

Map<KClass<*>, Middleware<AppState, Action, Action>?

[Evan R.]

Define the middleware interface as interface Middleware<S: State, out A: Action, out A2: Action> ...

[Evan R.]

Because you state that A and A2 are out generics and have a base type of Action, it means you can put Middleware instances into a map and the A and A2 types can specifically be subtypes of Action

[streetsofboston]

^^^ A should be contra-variant (in) at best (and A2 can be co-variant (out))

[carlos cdmp]

OOk thanks :))

[streetsofboston]

Or all of them can be invariant (no in nor out) if you are not interested in the assignability of sub-types of Middleware based its type-parameters A and A2 to a plain Middleware type.