Translate constructor of structs

[sgade]

Is your feature request related to a problem? Please describe.

Using a struct to manage data, in Swift you can add an initialiser for storing the properties:

public struct FoodSpecial {
    public let id: String
    
    public let title: String
    public let subtitle: String
    public let price: Double
    
    public init(title: String, subtitle: String, price: Double) {
        self.id = UUID().uuidString
        
        self.title = title
        self.subtitle = subtitle
    }
}

this is translated by Gryphon to a data class with a constructor:

data class FoodSpecial(
    val id: String,
    val title: String,
    val subtitle: String,
    val price: Double
) {
    constructor(
        title: String,
        subtitle: String,
        price: Double)
    {
        this.id = UUID().toString()
        this.title = title
        this.subtitle = subtitle
        this.price = price
    }
}

Describe the solution you'd like

According to my Kotlin knowledge, the best way to implement the behavior would be to use default values. See this example:

data class FoodSpecial(
    val id: String = UUID().toString(),
    val title: String,
    val subtitle: String,
    val price: Double
)

Describe alternatives you've considered

As data classes cannot have constructors, this seems to be the best way to approach this.

Additional context

none

[sgade]

This actually leads to Kotlin build errors: Primary constructor call expected and Val cannot be reassigned.

[vinivendra]

Hi @sgade, thanks for filing this issue (and the others).

We still need to figure out a more comprehensive solution for initializers everywhere, but perhaps we can find (or implement) a solution for your use case in the meantime.

Would you be OK with using something like this in Swift?

public struct FoodSpecial {
    public let id: String = "a"
    public let title: String
    public let subtitle: String
    public let price: Double
}

// OK
let food = FoodSpecial(title: "", subtitle: "", price: 0)

// Not allowed by Swift:
// let otherFood = FoodSpecial(id: "2", title: "", subtitle: "", print: 0)

This currently gets translated to

data class FoodSpecial(
    val id: String = "a",
    val title: String,
    val subtitle: String,
    val price: Double
)

fun main(args: Array<String>) {
    val food: FoodSpecial = FoodSpecial(title = "", subtitle = "", price = 0.0)
}

which works in Kotlin.

If this doesn't work for you, let me know why and we'll figure something out.

[sgade]

Thanks for your quick reply!
Your suggestions fine works for me and is suitable at the moment, so I'll go with that.

I like the idea of using warnings to inform the user that a certain feature is not (yet) support. Do you think it would make sense do add a warning for this case? It might refer the user to using the possible fix you suggested here.

[vinivendra]

Yeah, I don't think we currently support any initializers in structs, so it would make sense to raise warnings when we find them.

[sgade]

I just noticed again why I was using explicit initialisers: The structs I am translating are located in a Swift package and therefore need an explicit public init to be accessible by my app project.

Now I could create something business-y like Factories. I guess in my particular case there is not disadvantage in using classes instead.

[vinivendra]

Ok! I'm gonna implement the warnings for now, but I'll leave this issue open until the initializers are properly fixed.

You could probably ignore your Swift initializer with // gryphon ignore and manually add a Kotlin constructor with // gryphon insert: (like we do here) if you prefer to use structs.