I remember kmm used to have some restrictions havi...

# multiplatform

[Anton Afanasev]

I remember kmm used to have some restrictions having default arguments and swift/objective-c. Is it still a case? Recently I tried that out and it seem that kotlin native compiler does respsect function with default argument.
So this:

fun sendMessage(text: String, customAttributes: Map<String, String> = emptyMap())

translated to this:

func sendMessage(text: String, customAttributes: [String : String] = [:])

Am I seeing write?

kotlin 1.6.10

[kpgalligan]

func sendMessage(text: String, customAttributes: [String : String] = [:])

That’s swift. The Kotlin compiler would be generating objc. Where do you see that? Default parameters aren’t available, unless something I’m not aware of happened.

[Anton Afanasev]

That is really odd.
I would expect to see Kotlin compiler generating obj-c as well. Instead, I am seeing a swift generated code...
I see that by using the regular XCode Jump To Definition tool. Thats weird...

[Anton Afanasev]

Interestingly, if I pass an empty map as a default argument swift would not force me to provide a value for this argument. So I can do something like that:

common:

fun sendMessage(text: String, customAttributes: Map<String, String> = emptyMap())

ios/swift

client.sendMessage(text: "Hello")

No warnings or errors.

But that only work with emptyMap. If I go and define some other default argument, lets say:

fun sendMessage(text: String, customAttributes: Map<String, String> = mapOf("a" to "b")

They default argument will be ignored and the value of customAttributes in that case will be empty map as well.
Is it possible that K\N compiler always treat a map argument as an emptyMap, unless actual value is provided?

[Rick Clephas]

Cool! Didn’t know about this. This is actually part of the Swift - ObjC interop:

An argument whose Objective-C type is an NSDictionary defaults to nil if the NSDictionary is nullable and [:] if the NSDictionary is non-nullable, under the following conditions:

• the argument label contains the word “options” or “attributes”, or the two words “user info” one after another

• the argument label is empty and the method base name ends with the word “options” or “attributes”, or the two words “user info” one after another

https://github.com/apple/swift/blob/main/docs/CToSwiftNameTranslation.md#default-argument-values

[Rick Clephas]

Another nice one is:

A final argument that is a nullable function or block pointer defaults to nil.

fun doSomething(block: (() -> Unit)?) = TODO()

can be called from Swift like:

doSomething() // default nil
doSomething { }

[kpgalligan]

Huh, interesting

[kpgalligan]

I didn’t know that either

[Anton Afanasev]

Thanks @Rick Clephas and @kpgalligan
Thats a nice thing to know. Sounds like some sort of easter egg