How Can Recursive Type Constraints Be Achieved in Go Generics Using Interfaces?

Recursive Type Constraints with Predefined Types

In Go generics, specifying a type constraint on a generic type using an interface is possible. However, it can be challenging to enforce the implementation of a method with an argument of the generic type.

To achieve recursive type constraints in Go generics, we can define an interface with a method that takes the generic type as an argument:

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

We can then create a function that takes two arguments of type Lesser[T] and returns a boolean:

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

Now, custom types that implement the Lesser[T] interface can be used in the function:

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
}

The Lesser[T] constraint ensures that the types provided to the IsLess function implement a Less method that takes an argument of the same type. This allows for recursive type constraints without the use of type literals.

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