Thanks Matthew. That’s a quite elaborated proposal! I’ll be sure to keep an eye on it as we move forward.
Dimitri On 15 Feb 2017, at 16:57, Matthew Johnson <[email protected]<mailto:[email protected]>> wrote: Hi Dimitri, You may be interested in taking a look at a proposal I introduced about a year ago which was deferred. Memberwise initialization is a topic we intend to revisit eventually. This may happen in phase 2 of the Swift 4 effort, or may not happen until Swift 5. https://github.com/apple/swift-evolution/blob/master/proposals/0018-flexible-memberwise-initialization.md Matthew On Feb 15, 2017, at 9:38 AM, Dimitri Racordon via swift-evolution <[email protected]<mailto:[email protected]>> wrote: 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 _______________________________________________ swift-evolution mailing list [email protected]<mailto:[email protected]> https://lists.swift.org/mailman/listinfo/swift-evolution
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