Hello I have a question regarding generics I would like to s kotlinlang #getting-started

Hello, I have a question regarding generics. I would like to store lambdas into a Map. And those lambdas have a generic parameter. So, I did this simple code

typealias AFunction<R, A> = suspend (args: A) -> Result<R>

val functionMap = mutableMapOf<String, AFunction<Any?, Any?>>()

fun <R, A> addFunctionToMap(name: String, function: AFunction<R, A>) {
    functionMap[name] = function
}

For some reasons, it doesn’t compile (on the line

functionMap[name] = function

) as it is saying it was expecting a type

AFunction<Any?, Any?>

and not

AFunction<R, A>

. I tried modifying a bit the

addFunctionToMap

function like this:

fun <R: Any?, A: Any?> addFunctionToMap(name: String, function: AFunction<R, A>) {
    functionMap[name] = function
}

But, it doesn’t change the compiler error. Do you have an idea how I could solve that? 🙏

Timo Gruen

02/21/2021, 6:25 PM

Try to use „out Any?“ on definition side inside of just „Any?“

Ruckus

02/21/2021, 7:06 PM

MutableMap<String, AFunction<*, *>>

Melvin Biamont

02/21/2021, 7:27 PM

MutableMap<String, AFunction<*, *>>

works .. but Then if I try to call the function in it, the compiler tells me the parameter is of type

Nothing

(because of the Star projection).

Ruckus

02/21/2021, 7:42 PM

Of course not. Storing random generic functions in a map isn't type safe. You either need to cast it, or store type information with the functions somehow.

(or both)

Matteo Mirk

02/22/2021, 2:22 PM

What you need to solve the lack of type-safety here is the Typesafe Heterogeneous Container pattern: https://stevewedig.com/2014/08/28/type-safe-heterogenous-containers-in-java/

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