How to make a generic parameter "optional/default"...

# getting-started

[David Kubecka]

How to make a generic parameter "optional/default" when it's needed just in an optional function parameter?

fun <T> foo(arg: String, handler: (String) -> T = {}): T

I would like to define the function so that it's possible to call it with a default parameter:

foo("abc")

[David Kubecka]

Ideally I would like to avoid specifying the type param in this case, but even calling foo<Unit>("abc") would be fine.

[David Kubecka]

The trouble is that I don't know how to make the default parameter. Currently it doesn't compile with:

[TYPE_MISMATCH] Type mismatch.
Required:
T
Found:
Unit

[David Kubecka]

(Of course, the real-life function is more complex so e.g. simply using overloading isn't a good option.)

[Joffrey]

It's not possible to have a default value for a generic type parameter, so there is no such thing as a "default `T`" for you. Now, your second parameter is a function that's supposed to return T. If you want to provide a default value for that one, it has to match any possible T that someone would use on the call site, which is generally not possible either, unless your default lambda just throws and returns Nothing 🙂

[Joffrey]

The best option for you is to define a non-generic overload:

fun foo(arg: String): Unit = foo<Unit>(arg) {}

fun <T> foo(arg: String, handler: (String) -> T): T {
    ...
}

[Joffrey]

Of course, the real-life function is more complex so e.g. simply using overloading isn't a good option

Then please show the real-life function 😉

[David Kubecka]

Ah, I for some reason thought that the overload must be complicated than that 😄

[David Kubecka]

It's just somewhat visually/syntactically disappointing seeing

fun foo(arg: String): Unit = foo(arg) {}

and not being able to convert this to a default parameter 🙂

[Joffrey]

Yeah, I know, but when you think about why, it kinda makes sense. So I made my peace with it :)

[David Kubecka]

Sure, it totally makes sense.

[Tobias Suchalla]

You can also make the block nullable:

fun <T> foo(arg: String, handler: ((String) -> T)? = null): T

But I guess that won't help you to return T from that function...