# Sealed interface with inner classes that implements super interface and its super interface?

**URL:** <https://discuss.kotlinlang.org/t/sealed-interface-with-inner-classes-that-implements-super-interface-and-its-super-interface/25619>\
**Category:** Language Design\
**Created:** [September 6, 2022, 9:36pm UTC](https://discuss.kotlinlang.org/t/sealed-interface-with-inner-classes-that-implements-super-interface-and-its-super-interface/25619 "2022-09-06T21:36:09Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![mrpug](https://avatars.discourse-cdn.com/v4/letter/m/e5b9ba/32.png) [@mrpug](https://discuss.kotlinlang.org/u/mrpug)\
**Post date:** [September 6, 2022, 9:36pm UTC](https://discuss.kotlinlang.org/t/sealed-interface-with-inner-classes-that-implements-super-interface-and-its-super-interface/25619/1 "2022-09-06T21:36:10Z")

</div>

I’m wanting to know if it’s possible to declare a sealed interface, and to then declare a sealed class that implements that interface that “re-aliases” the properties of the interface so that they have more domain-specific names.

E.g. I am doing the Either pattern from functional programming, but I want to make a subclass that uses terms other than the functional programming “left” and “right” names. This is for convenience.

Here’s my current attempt:

```auto
// Either.kt
sealed interface Either<A, B> {
    open class Left<A, B>(open val left: A) : Either<A, B>
    open class Right<A, B>(open val right: B) : Either<A, B>
}

```

```auto
// ContentEither.kt
sealed interface ContentEither<T> : Either<Throwable, T> {
    data class Error<Throwable, T>(val error: Throwable) : Either.Left<Throwable, T>(error) {
        override val left: Throwable = this.error
    }
    data class Content<E, T>(val content: T) : Either.Right<E, T>(content) {
        override val right: T = this.content
    }
}

```

The problem is, `Error` is not known to implement `ContentEither` so then I get errors like this when I try to use it

```auto
Type mismatch.
Required:
ContentEither<T>
Found:
ContentEither.Content<???, T>

```

If I try to say that it does implement `ContentEither`, I then get this error:

```auto
sealed interface ContentEither<T> : Either<Throwable, T> {
    data class Error<Throwable, T>(val error: Throwable) : Either.Left<Throwable, T>(error), ContentEither<T>() {
        override val left: Throwable = this.error
    }
    data class Content<Throwable, T>(val content: T) : Either.Right<Throwable, T>(content), ContentEither<T> {
        override val right: T = this.content
    }
}

```

```auto
Type parameter A of 'Either' has inconsistent values: Throwable#1 (type parameter of org.me.ContentEither.Error), kotlin.Throwable

```

Is there a way to specify that these are the same Throwables? Is what I am trying to do possible in Kotlin?

---

<div class="post-metadata">

**Author:** ![Beholder](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/beholder/32/2078_2.png) [@Beholder](https://discuss.kotlinlang.org/u/Beholder)\
**Post date:** [September 8, 2022, 6:36pm UTC](https://discuss.kotlinlang.org/t/sealed-interface-with-inner-classes-that-implements-super-interface-and-its-super-interface/25619/2 "2022-09-08T18:36:28Z")

</div>

In your code

```auto
    data class Error<Throwable, T>(val error: Throwable) : Either.Left<Throwable, T>(error), ContentEither<T>() {
// ^^^^^^^^^ this is not name of class, but name of type variable

```

Also, same references is stored twice (in base class and in derived), wasting some memory.

Correct code will be

```auto
sealed interface ContentEither<T> : Either<Throwable, T> {
    data class Error(val error: Throwable) : Either.Left<Throwable, Nothing>(error), ContentEither<Nothing> {
        override val left: Throwable get() = error
    }
    data class Content<T>(val content: T) : Either.Right<Throwable, T>(content), ContentEither<T> {
        override val right: T get() = this.content
    }
}

```

---

<div class="post-metadata">

**Author:** ![mrpug](https://avatars.discourse-cdn.com/v4/letter/m/e5b9ba/32.png) [@mrpug](https://discuss.kotlinlang.org/u/mrpug)\
**Post date:** [September 8, 2022, 8:53pm UTC](https://discuss.kotlinlang.org/t/sealed-interface-with-inner-classes-that-implements-super-interface-and-its-super-interface/25619/3 "2022-09-08T20:53:40Z")

</div>

Thanks!