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

[Matthew Johnson]

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 <swift-evolution at swift.org> 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
> 
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