ExpressibleByStringLiteral provides a secret initialiser?

Tuesday, 05 March 2024

struct Blob {
  let value: String
}

let blob = Blob("string")

Missing argument label ‘value:’ in call

extension Blob: ExpressibleByStringLiteral {
  init(stringLiteral value: String) {
    self.value = value
  }
}

This turns out to be helpful to clarify a type.

struct Foo {

  private let value: String

  init(_ blob: Blob) {
    self.value = "blob:" + blob.value
  }

  init(_ value: String) {
    self.value = value
  }
}

let foo1 = Foo("string")
let foo2 = Foo(Blob("string"))

This is not an initializer!

If we add .init to force the use of an initializer:

Blob.init("string")

This will provide a failure that there is no matches for the initializer:

No exact matches in call to initializer

So what is this syntax doing?