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

*

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).

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….

Then how can i write

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

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

?

Define the middleware interface as

interface Middleware<S: State, out A: Action, out A2: Action> ...

^^^

A

should be contra-variant (

in

) at best (and

A2

can be co-variant (

out

))

OOk thanks :))

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.