Is there a way to use

putAll

on a mutable map, passing it another map and a value transformation function?

putAll

allows you to put all entries of another map into this map, but doesn't let you transform them on the fly.

mapTo

allows you to transform the entries but you can only write them to a Collection, not to a MutableMap. Is there a method that does what the commented-out line does?

fun main() {
    val map1 = mapOf(1 to "one", 2 to "two", 3 to "three")
    val map2 = buildMap {
        put(4, "fourfour")
        put(5, "fivefive")

        // map1.mapTo(this) { (k, v) -> k to v + v } DOESN'T COMPILE

        // instead we have to use:
        putAll(map1.toMutableMap().apply { replaceAll { _, v -> v + v } })
        // or:
        putAll(map1.map { (k, v) -> k to v + v })
    }
    println(map2)
}

putAll(map1.mapValues { (_, v) -> v + v })

?

Thanks, Paul. Your suggestion has led me to exactly what I'm looking for:

mapValuesTo

.

map1.mapValuesTo(this) { (k, v) -> v + v }

Yeah, just realized you were more asking for direct mutation of the temp

buildMap

backing map