is there a way to make a `Map` that's guaranteed t...

# announcements

[LastExceed]

is there a way to make a Map that's guaranteed to be exhaustive, so the return type of get() isn't nullable? (similar to how when can skip else when it's exhaustive)

[Rob Elliot]

It’s an enum class, isn’t it?

[christophsturm]

or a map with an enum as key

[LastExceed]

the key is an enum. unfortunately i can't just make the associated value a property of that enum

[Rob Elliot]

A when will have the same effect as an exhaustive map. Make it an extension function and it’s almost a property of the enum.

[LastExceed]

the values of the map are computed, using a when would mean re-computing the values on every lookup

[Derek Ellis]

Something like this? https://quickbirdstudios.com/blog/non-empty-lists-kotlin/ (but for Map )

[LastExceed]

not really. i guess i have to wrap the map into a container that does a non-null assertion internally

[mkrussel]

Map has a getValue method that can be used when you know the key will be in the map.

[LastExceed]

oh thats good to know, ty

[ephemient]

.withDefault() has the same issue, the typesystem doesn't know that .get() can't return null from it

[ephemient]

in theory you could define

class MyEnumMap(private val delegate: Map<MyEnum, String>) : Map<MyEnum, String> by delegate {
    override fun get(key: MyEnum): String = delegate.getValue(key)
}

but there's some issues with delegation + override in earlier Kotlin versions, so do some investigation to check if it works for you. https://blog.jetbrains.com/kotlin/2020/07/kotlin-1-4-m3-generating-default-methods-in-interfaces/#:~:text=fixing%20an%20issue%20with%20delegates

[Joost Klitsie]

@Derek Ellis that is a great link! I think it would be a great addition to the standard library 🙂