Swift loses the type of a Kotlin Flow, because Flow is an interface

The gap

I learned that Swift sees the values of a Kotlin Flow<User> as Any?, and accepted it as a fact. What I was missing was the reason, so I couldn't predict which other types would lose information too.

The pieces

• Kotlin/Native doesn't produce Swift. It produces a framework with an Objective-C header, and Swift reads that header.
• Objective-C has only "lightweight generics", and they can be defined on classes, not on interfaces (protocols) or functions.
• A Kotlin interface becomes an Objective-C protocol.
• Flow<out T> is an interface in kotlinx.coroutines, so it becomes a protocol, and the T has nowhere to go.

The click

The Objective-C header is a restaurant menu that Swift orders from. The menu can print "soup of the day: tomato" next to a dish (a class), but for a protocol it can only print "soup". So a generic class such as Box<User> keeps its type in Swift, while a Flow<User> arrives as "a flow of something", and Swift has to cast.

The same rule predicts the rest: anything generic that isn't a class, such as a generic function, loses its type parameter in the header too.

How I checked it

I read the Kotlin documentation on Objective-C interop: "Generics can only be defined on classes, not on interfaces (protocols in Objective-C and Swift) or functions." The kotlinx.coroutines API reference declares Flow as interface Flow<out T>.

I didn't build an iOS framework to look at the generated header; that needs a Mac. So this card is a seedling until I do.

Check yourself

Sources

• Kotlin docs: Interoperability with Swift/Objective-C, generics
• kotlinx.coroutines API reference: Flow