How Can We Implement a Generic Either Type in Go Using Generics and Optionals?

Generic Either Type Implementation in Go

In Go 1.18, generics offer promising possibilities for expressing complex concepts. One such concept is the Either type, which can represent a value of either type A or type B. This article explores potential ways to implement an Either type within Go's new generics paradigm.

The challenge arises from Go's restriction on interface methods having no type parameters. This hurdle prevents the direct implementation of an Either interface.

Emulating Either with Optional

One approach is to adapt the functional programming concept of Option. An Optional[T] interface can represent a value of type T or the absence of a value. By leveraging Go's generics, we can extend this idea to create an Optional for either A or B.

type Optional[T any] interface {
    get() (T, error)
}

func CreateNone[T any]() Optional[T] {
    return None[T]{}
}

func CreateSome[T any](data T) Optional[T] {
    return Some[T]{data}
}

Implementing Either

With Optional established, we can now construct an Either type that leverages it.

type Either[A, B any] interface {
    is_left() bool
    is_right() bool
    find_left() Optional[A]
    find_right() Optional[B]
}

Left and Right structs represent values of type A and B, respectively. is_left and is_right indicate which type the Either holds. find_left and find_right provide the Optional wrappers for the respective values.

type Left[A, B any] struct {
    data A
}

func left[A, B any](data A) Either[A, B] {
    return Left[A, B]{data}
}

type Right[A, B any] struct {
    data B
}

func right[A, B any](data B) Either[A, B] {
    return Right[A, B]{data}
}

Usage

The implemented Either type can be utilized as follows:

var e1 Either[int, string] = left[int, string](4143)
var e2 Either[int, string] = right[int, string]("G4143")

if e1.is_left() {
    if l, err := e1.find_left().get(); err == nil {
        fmt.Printf("The int is: %d\n", l)
    }
}

This approach effectively emulates the Either type while respecting Go's interface constraints. It enables the expression of values that may belong to one of two types, offering greater flexibility and type safety in Go code.

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