Is it possible to have partial inference? I've got...

# getting-started

[Klitos Kyriacou]

Is it possible to have partial inference? I've got something like:

inline fun <K, reified V> Map<out K, *>.filterValueIsInstance(): Map<K, V>

and it needs to be called like this:

mapOf('i' to 1, 'j' to 2, 'l' to 3L, 's' to 4.toShort()).filterValueIsInstance<Char, Int>()  // == mapOf('i' to 1, 'j' to 2)

Is it possible to define it in such a way that the compiler infers what type the keys are, and just needs the target type, like this:

mapOf('i' to 1, 'j' to 2, 'l' to 3L, 's' to 4.toShort()).filterValueIsInstance<Int>()  // == mapOf('i' to 1, 'j' to 2)

[Javier]

The only way is creating a filterValueIsInstance which has only a type parameter.

Have you tried something like

inline fun <reified V> Map<*, *>.filterValueIsInstance(): Map<*, V>

[Youssef Shoaib [MOD]]

I wish that was possible. The closest you can get is:

myMap.filterValueIsInstance<_, Int>()

[Tomer Arazy]

You can do:
inline fun <K, reified V : Any> Map<out K, *>.filterValueIsInstance(c: KClass<V>): Map<K, V>
And then call it like:
mapOf('i' to 1, 'j' to 2, 'l' to 3L, 's' to 4.toShort()).filterValueIsInstance(Int::class)
I'm not a big fan of this syntax but don't think there is a better alternative.

[Klitos Kyriacou]

Interesting, thanks. And if you have a default parameter value – (c: KClass<V> = V::class) – then the caller has a choice of syntax. 😀

[Youssef Shoaib [MOD]]

If you do it that way, then you might want to use a lighter-weight way to carry around the type. I've used this pattern before the _ existed for inferring type arguments:

fun main() {
    mapOf('i' to 1, 'j' to 2, 'l' to 3L, 's' to 4.toShort()).filterValueIsInstance(type<Int>())
}
inline fun <K, reified V> Map<out K, *>.filterValueIsInstance(type: TypeWrapper<V> = type()): Map<K, V> = TODO()

sealed interface TypeWrapper<out T> {
    companion object IMPL: TypeWrapper<Nothing>
}

fun <T> type(): TypeWrapper<T> = TypeWrapper.IMPL

[Javier]

Why not just filterValueIsInstance<Int>()?

[Youssef Shoaib [MOD]]

@Javier because we want to preserve the key type of the map.