CODEWITHSUNDEEP
genericskotlin
juliancadi
juliancadi

Reputation: 1024

Global generic value in Kotlin

Given the simple class:

data class Optional<T>(val value: T?)

I can define generic function and value extensions as follows:

fun <T> T?.makeOptional() = Optional(this)  // Compiles ✅
val <T> T?.optional get() = Optional(this) // Compiles ✅

I can also define global generic functions:

fun <T> makeOptionalNull() : Optional<T> = Optional(null) // Compiles ✅

However I CAN'T define an equivalent global generic value to this last function:

val <T> optionalNull : Optional<T> = Optional(null) // Compilation Error ❌
//   ^ Type parameter of a property must be used in its receiver type

A global non-generic value is totally fine though:

val stringNull : Optional<String> = Optional(null) // Compiles ✅

How can I create a global generic value similar to the one above? Is it even possible?

Upvotes: 1

Views: 356

Answers (1)

Mikhail Burshteyn
Mikhail Burshteyn

Reputation: 5002

Your problem can be solved the following way:

data class Optional<out T>(val value: T?) // <- note the 'out' modifier here

val optionalNull: Optional<Nothing> = Optional(null)

// and use it as follows:
val myOptionalVariable: Optional<String> = optionalNull // assignable thanks to 'out' modifier

Upvotes: 1

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