Compare

type Comparable = class
    def (>) (rhs : Self) : bool
    def (<) (rhs : Self) : bool
    def (>=) (rhs : Self) : bool
    def (<=) (rhs : Self) : bool

Class is a contract. It does not exist during runtime and cannot be casted to, but is useful as a constraint on a generic type parameter.

Self is the type of an instance on which a method is invoked.

type Compare = class
    def compare (other : Self) : Ordering

    def (>) (rhs : Self) =
        let result = compare rhs
        result == Ordering/Greater

    def (>=) (rhs : Self) =
        let result = compare rhs
        result <> Ordering/Less

    def (<) (rhs : Self) =
        let result = compare rhs
        result == Ordering/Less

    def (<=) (rhs : Self) =
        let result = compare rhs
        result <> Ordering/Greater

    is Comparable

Class Compare provides comparison operators in exchange for implementing a single compare method.

type (T1, T2) u v where T1 : Comparable
                        T2 : Comparable
    def compare (other : Self) = when
        u > other.u -> Ordering/Greater
        u < other.u -> Ordering/Less
        v > other.v -> Ordering/Greater
        v < other.v -> Ordering/Less
        else -> Ordering/Equal

    is Compare

We extend a tuple of two items T1 and T2 that are Comparable. u and v are the item names to use in methods. Tuple belongs to both Comparable and Compare classes.