How Can I Define Recursive Type Constraints in Go2 Generics Using Interfaces?

Defining a Recursive Type Constraint Using a Predefined Interface

In Go2 generics, interfaces can be used to specify type constraints on generic types. However, the current draft does not provide a way to force the implementation of a method with an argument of the generic type itself.

To overcome this limitation, consider the following approach:

Define a Recursive Interface:

type Lesser[T any] interface {
    Less(T) bool
}

Define a Function with a Generic Type Parameter Constrained by the Recursive Interface:

func IsLess[T Lesser[T]](x, y T) bool {
    return x.Less(y)
}

Usage:

type Apple int

func (a Apple) Less(other Apple) bool {
    return a < other
}

type Orange int

func (o Orange) Less(other Orange) bool {
    return o < other
}

func main() {
    fmt.Println(IsLess(Apple(10), Apple(20)))   // true
    fmt.Println(IsLess(Orange(30), Orange(15))) // false
}

Explanation:

The type constraint T Lesser[T] ensures that the generic type T must implement the Less method with an argument of type T. This allows for recursive type constraints.

This approach enables you to define custom types like Apple and Orange that implement their own Less methods, fulfilling the recursive constraint and enabling the usage of the IsLess function with those custom types.

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