While subtyping is often ampliative, that is, a subtype contains all the properties of its supertype(s), it’s often useful to remove an inherited property from a subtype. For example, a complex has a real part and an imaginary part. Every real number is also a complex number but its imaginary part is always equal to zero so we don’t want this property to take up space in the instances. However we can’t remove the imaginary part altogether because it might be used by code working with complex numbers.What one can do is assign the property a fixed value:
type Complex { property real Float property imag Float func _add(x Complex) Complex { return new Complex { real=self.real+x.real, imag=self.imag+x.imag } } func _mul(x Complex) Complex { return new Complex { real=self.real*x.real-self.imag*x.imag, imag=self.real*x.imag+self.imag*x.real } } func description() String { return self.real.description() + "+" + self.imag.description() + "i" }}type Real : Complex { restrict imag Float = 0.0}main { var x = new Complex { real=2.0, imag=3.0 } var y = new Real { real=2.0 } return (x*y).description()}Note that theReal type now has only one stored property, but the methods in theComplex type can still be used with the subtype.
NB: The pseudocode can be tried out using the Funcy app, which can be downloaded for free fromApple’s App Store (iOS/macOS),Google Play (Android) orAmazon Appstore. The code to be executed must be placed in amain {} block.