I've got a sealed interface `NbtTag` that's implem...

# announcements

[Ben Woodworth]

I've got a sealed interface NbtTag that's implemented by NbtInt, NbtString, NbtList<T : NbtTag>, etc. (for a kotlinx.serialization format library)

NbtList can only contain one type of tag (so T can't be NbtTag), and I'm wondering if there's a better way to restrict that. Here's basically what I'm doing right now (which seems overly verbose):

class NbtList<out T : NbtTag> internal constructor(
    val content: List<T>
) : NbtTag, List<T> by content

// Pseudo constructors to limit T to concrete types
fun NbtList(content: List<NbtInt>): NbtList<NbtInt> = NbtList<NbtInt>(content)
fun NbtList(content: List<NbtString>): NbtList<NbtString> = NbtList<NbtString>(content)
fun NbtList(content: List<NbtList<*>>): NbtList<NbtList<*>> = NbtList<NbtList<*>>(content)
// plus 9 more for the remaining tag types...

Are there any other ways I can accomplish this and limit the constructor/content?

[Dominaezzz]

I'd probably enforce this in the encoder/decoder. I don't think these restrictions are going to be easy to work with imo.

[Ben Woodworth]

It's a restriction with the binary format so my thinking was to model the class similarly in a type-safe way (so all NbtList instances valid for serialization). Plus checks in the encoder in case any custom @Serializable types are invalid just in case

[Paul Griffith]

could you have a sealed interface NbtItem : NbtTag and have NbtList accept NbtItem as the constraint?

[ephemient]

runtime hack, but this may work:

Json.encodeToString(NbtList(NbtInt(1), NbtInt(2))) // ok
Json.encodeToString(NbtList(NbtInt(1), NbtString("2"))) // throws
Json.encodeToString(NbtList(NbtList(NbtInt(1)), NbtList(NbtInt(2)))) // ok
Json.encodeToString(NbtList(NbtList(NbtInt(1)), NbtList(NbtString("2")))) // throws

Untitled.kt