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Donovan LaDuke - Developer

Removing Ambiguity in Kotlin Function by Reference

Featured in Android Weekly Issue 604

In Kotlin, functions are a first-class citizen and can be stored in variables and referenced later. This means you can reference a function on a class by instance or by reference.

val myInt = 10
val funcRef = myInt::toString // Function reference
funcRef() // Invokes the function from the reference
myInt.toString() // Invokes the function by instance 

This is useful in a lot of scenarios, however an issue can arise if you try to reference a function by reference when there are overloads on that function name.

data class AdditionClass(val str: String, val num: Int) {
  fun add(addStr: String): String {
    return str + addStr
  fun add(addNum: Int): Int {
    return num + addNum

val addition = AdditionClass("Hello", 10)

// Compile Error!
val add = addition::add 

While the solution to just "remove the overload" may seem obvious, that wouldn't solve the case where the code can't be changed such as with an external library. This leaves the only option being to specify the type explicitly. Note that the type of the variable must be specified, the as operator will not work as it isn't applied until run time.

// Success!
val addInt: (Int) -> Int = addition::add 

// Still a Compile Error!
val addString = addition::add as (String) -> String

Conclusion #

Here is a short gist that shows the problem and proposed solution, feel free to explore it. Hopefully in the future the Kotlin team will expose a way to request a specific method by the parameter types which would remove this issue in its entirety. For now, specifying the type should resolve any jams. Until next time, thanks!

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