[swift-evolution] a few initializer proposals

[Mike Kluev]

hi. here are a few somewhat related initializer proposals for your
consideration.
they are designed as "opt-ins" so shall not break existing code.

***** proposal 1 *****
have an ability to keep default initializers in structs

struct S {
    var int: Int
    var float: Float
    var bool: Bool
}

here default initializer is currently generated:

struct S {
    var int: Int
    var float: Float
    var bool: Bool

//  autogenerated:
//  init(int: Int, float: Float, bool: Bool) {
//      self.int = int
//      self.float = float
//      self.bool = bool
//  }
}

so we can write:

let s = S(int: 0, float: 0, bool: false)

so far so good. now i want to add a "convenience" initializer. i don't want
the default initializer to go away though:

struct S {
    var int: Int
    var float: Float
    var bool: Bool

    /* convenience */ init(x: Int) {
        self.init(int: 0, float: 0, bool: false) // *** ERROR. this
initializer is gone
    }
}

let s = S(x: 0)
let s = S(int: 0, float: 0, bool: false) // *** ERROR. this initializer is
gone

this may be convenient in some cases but not others.
proposal: introduce an opt-in construct to keep the default initializer
intact:

struct S {
    var int: Int
    var float: Float
    var bool: Bool

    default init // explicit opt-in. suggest a better name

    /* convenience */ init(x: Int) {
        self.init(int: 0, float: 0, bool: false) // still ok
    }
}

let s = S(x: 0) // ok
let s = S(int: 0, float: 0, bool: false) // still ok


***** proposal 2 *****
have the default initializer to have default nil values for all optional
parameters in structs

struct S {
    var int: Int
    var float: Float?
    var bool: Bool

//  autogenerated:
//  init(int: Int, float: Float? = nil, bool: Bool) {
//      self.int = int
//      self.float = float
//      self.bool = bool
//  }
}

in the extreme case:

struct S5 {
    var int: Int?
    var float: Float?
    var bool: Bool?

//  autogenerated:
//  init(int: Int? = nil, float: Float? = nil, bool: Bool? = nil) {
//      self.int = int
//      self.float = float
//      self.bool = bool
//  }
}

usage:
S(float: 3)
S()
S(int: 0, float: 0, bool: false)

note that this is still "opt-in" as the old code will be using the full
form anyway and not break.

***** proposal 3 *****
introduce a similar opt-in default initialiser for classes:

class C {
    var int: Int
    var float: Float?
    var bool: Bool
}

let c = C(int: 0, float: 0, bool: false) // *** ERROR

class C {
    var int: Int
    var float: Float?
    var bool: Bool

    default init // explicit opt-in. suggest a better name

//  this is autogenerated:
//  init(int: Int, float: Float? = nil, bool: Bool) {
//      self.int = int
//      self.float = float
//      self.bool = bool
//  }
}

let c = C(int: 0, bool: false) // ok


***** question 4 *****

this one is not a proposal, rather a question. probably there is a good
reason for it, i just want to know it: why there is no default initializer
for classes similar to what we have for structs?


=================================
thoughts and comments on any of these?

Mike
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