Why is the result of the computeIfAbsent lambda non-nullable?

fstanis · October 12, 2020, 2:16pm

I’m not sure if this is a bug or a lack of understanding on my part, but I was surprised with the following null behavior:

val testMap = mutableMapOf<String, String>()
testMap.computeIfPresent("test") { key, value ->
    null // this works
}
testMap.computeIfAbsent("test") { key ->
    null // this doesn't: "Null can not be a value of a non-null type String"
}

Java docs for computeIfAbsent specifically state “[i]f the function returns null no mapping is recorded”, so returning null in that lambda should be valid behavior. Does Kotlin have a different implementation of computeIfAbsent or is this perhaps an issue with how the lambda type is interpreted?

akurczak · October 12, 2020, 3:13pm

It seems to be done on purpose by Kotlin team:

github.com

JetBrains/kotlin/blob/36d2129fd3fc45524d34805ee86079df3b2d5166/core/compiler.common.jvm/src/org/jetbrains/kotlin/load/java/typeEnhancement/predefinedEnhancementInfo.kt#L109


      
              returns(BOOLEAN)
          }
          function("replaceAll") {
              parameter(JFBiFunction, NOT_PLATFORM, NOT_PLATFORM, NOT_PLATFORM, NOT_PLATFORM)
          }
          function("compute") {
              parameter(JLObject, NOT_PLATFORM)
              parameter(JFBiFunction, NOT_PLATFORM, NOT_PLATFORM, NULLABLE, NULLABLE)
              returns(JLObject, NULLABLE)
          }
          // while it is possible to return nullable value from lambda in computeIfAbsent,
          // we deliberately make it just NOT_PLATFORM V in order to have the return type V and not V?
          function("computeIfAbsent") {
              parameter(JLObject, NOT_PLATFORM)
              parameter(JFFunction, NOT_PLATFORM, NOT_PLATFORM, NOT_PLATFORM)
              returns(JLObject, NOT_PLATFORM)
          }
          function("computeIfPresent") {
              parameter(JLObject, NOT_PLATFORM)
              parameter(JFBiFunction, NOT_PLATFORM, NOT_PLATFORM, NOT_NULLABLE, NULLABLE)
              returns(JLObject, NULLABLE)

You can find some reasoning for such decision here:
https://youtrack.jetbrains.com/issue/KT-10982#focus=streamItem-27-2187715.0-0

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Why is the result of the computeIfAbsent lambda non-nullable?

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