Hi there, is it possible to do this in Kotlin some...

# announcements

[Gerard Bosch]

Hi there, is it possible to do this in Kotlin somehow? (it is just an example) I get error at call-site: Unresolved reference: of

sealed class Sealed {
    
    // Is it possible to define a factory method or smart constructor here?
    fun of(n: Int): Sealed = if (n > 0) Foo("test") else Bar(42)
    
    data class Foo(val str: String) : Sealed()
    data class Bar(val num: Number) : Sealed()
}

and why is the above not possible? thank you!

[Nir]

You would define such a function on an anonymous companion

[Nir]

Or you could just define a top level function outside the class

[Holden Easley]

^^ of is a member function of Seal so you need to make it accessible without an instance of Seal using one of the methods mentioned above

[Nir]

Right. Even if that worked, it wouldn't do what you probably want it to. Beyond that, there may be restrictions on sealed classes, I've never actually tried to use one like that so I'm not sure

[Nir]

A constructor cannot be polymorphic, I expect, it needs to know statically what type it's constructing

[Gerard Bosch]

It works with a top level function outside the clas yes, but I wanted to qualify the call as Sealed.of() if possible. The companion object did the job, thank you! 😃

[Nir]

np

[Nir]

btw, when you define the call operator for the companion object, those are actually called "smart constructors"

[Nir]

because it looks just like a constructor call (Sealed(foo) ) but it actually calls a function

[Nir]

so you can do things that you can't in a normal constructor, i.e. polymorphic return, generic

[Gerard Bosch]

Nice! thank you

[Gerard Bosch]

Happy to learn 🙂

[Nir]

Sorry, invoke, not call

[Gerard Bosch]

ah, yes invoke, that makes sense, I see

[Nir]

np 🙂

[todd.ginsberg]

Didn’t know that pattern had a name. Thanks Nir! :)

[ephemient]

operator fun invoke() - no magic without the right keywords 🙂

[ephemient]

you can also name a top-level function the same as the class, e.g.

fun Sealed(n: Int) = if (n > 0) Sealed.Foo("test") else Sealed.Bar(42)

[ephemient]

used in the standard library, List() etc.

[Gerard Bosch]

Oh, good idea, yes 🙂

[ephemient]

if you rely on anything private within the class though, only companion object will work, because that will have access

[Gerard Bosch]

I think I like even more than operator invoke or of()

[Nir]

Yeah, a top level function is ideal, it's the simplest, assuming you don't need the private access