Hello, I have a question regarding generics. I wo...

# getting-started

[Melvin Biamont]

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]

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

[Ruckus]

MutableMap<String, AFunction<*, *>>

[Melvin Biamont]

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]

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.

[Ruckus]

(or both)

[Matteo Mirk]

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/