[swift-evolution] Rules for structs default/memberwise initializers

[Dimitri Racordon]

Hello community!

While writing a Swift introduction tutorial for students, I’ve been stumbling upon the rules for struct default and memberwise initializers.
I failed to find explanations in Apple’s language guide, but as far as I could observe, I think the rules don’t fit interesting use-cases.

Here are the cases that I was able to identify (I hope you don’t mind millennials and their obligatory Pokemon references):

First, as documented in Apple’s guide, structs that doesn’t define any initializer and have no default values receive a memberwise initializer:

typealias Species = (number: Int, name: String)

struct Pokemon {
    let species: Species
    var level: Int
    var nickname: String
}

let bulby = Pokemon(species: (001, "Bulbasaur"), level: 1, nickname: "bulby")

Structs that define a default value for all their properties receive a default initializer:

struct Pokemon {
    let species: Species = (001, "Bulbasaur")
    var level: Int = 1
    var nickname: String = "bulby"
}

let bulby = Pokemon()

Now digging a bit deeper, I noticed that they also seem to receive an initializer for their non-constant properties:

let bulby = Pokemon(level: 1, nickname: "bulby")

If no value is provided for one (or several) of its variable properties, they receives an initializer for all their variable properties:

struct Pokemon {
    let species: Species = (001, "Bulbasaur")
    var level: Int = 1
    var nickname: String
}

let bulby = Pokemon(level: 1, nickname: "bulby")

Finally, if they're given a default value for their variable properties but not for their constant properties, they receive the full memberwise initializer only:

struct Pokemon {
    let species: Species
    var level: Int = 1
    var nickname: String = "bulby"
}

let bulby = Pokemon(species: (001, "Bulbasaur"), level: 1, nickname: "bulby")

If the two extreme cases sounds perfectly valid to me (no default value vs all default values), the mixed situations do not.
In particular, it seems strange that a struct without a default value for its constant property, but one for all its variable properties receives the memberwise initializer only. I guess that would be a common “mixed situation” case, yet the provided initializer is actually useless.

Receiving the full memberwise initializer is fine, but I would also expect to receive some kind of "partial memberwise” initializer for all properties (constants or variables) that are not defined:

struct Pokemon {
    let species: Species
    var level: Int = 1
    var nickname: String = "bulby"
}

let bulby = Pokemon(species: (001, "Bulbasaur”))
print(bulby)
// Prints "Pokemon(species: (1, "Bulbasaur"), level: 1, nickname: "bulby")"

Besides, that would avoid some tedious initializer definitions. Indeed, If I want to get the desired result, I have to write this kind of initializer:

struct Pokemon {
    let species: Species
    var level: Int = 1
    var nickname: String = "bulby"

    init(species: Species, level: Int? = nil, nickname: String? = nil) {
        self.species = species

        if level != nil {
            self.level = level!
        }

        if nickname != nil {
            self.nickname = nickname!
        }
    }
}

In addition to be rather wordy, it arguably destroys the purpose of defining a default value for variable properties in the first place, since imho this approach is clearer (unless maybe for some more complicated structs with multiple layers of initializer delegation):

struct Pokemon {
    let species: Species
    var level: Int
    var nickname: String

    init(species: Species, level: Int = 1, nickname: String = "bulby") {
        self.species = species
        self.level = level
        self.nickname = nickname
    }
}

Thanks.

Dimitri Racordon

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