Type-oriented programming/Self at the type level

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Just like the self variable refers contextually to the instance in whose method it’s used, the Self type variable contextually refers to the type. Let’s have a look at a simple example:

type A {
  sfunc create() Self {
    return new Self

type B : A {}

main {
  var obj = $B.create()
  return obj

The create method’s return type is Self so in the example above it refers to the B type because we called the type method using $B.create().

Using Self gives the type system considerable power since it squares well with type operators, as illustrated by the following example:

type Maybe[T] {
  property val T

  func description() String {
    return self.val.isNil() ? "none" : self.val.description()

  func map[U](f Func[T,U]) Self[U] {
    return new Self[U] {
      val = self.val.isNil() ? nil : f(self.val)

main {
  var x = new Maybe[Int] { val=3 }
  var y = x.map[Float](\(x Int) Float . x.asFloat())
  return y.description() + " " + y.typeName()

Note that the map method has a type argument and how new Self[U] is used. In this case Self is a contextual type operator.

NB: The pseudocode can be tried out using the Funcy app, which can be downloaded for free from Apple’s App Store (iOS/macOS), Google Play (Android) or Amazon Appstore. The code to be executed must be placed in a main {} block.