How Can We Implement an Either Type in Go, Given the Limitations of Generics in Interface Methods?

Implementing an Either Type in Go: Navigating Generics' Boundaries

The Problem and the Goal

With generics now a part of Go 1.18, many developers have set their sights on creating types that accurately represent complex concepts. A classic case is the 'Either[A, B]' type, which expresses the possibility of a value being one of two distinct types, A or B.

Interface Definition Hurdles

In defining an interface for an 'Either' type, a hurdle emerges due to the language's limitation of not allowing type parameters in interface methods. This restriction prevents us from expressing the 'Switch' method, which returns different types based on the value's variant.

A Creative Implementation

To overcome this issue, one could draw inspiration from functional programming languages and implement an 'Optional' type that encapsulates the ability to represent either a specific value or an absence. Building upon this, we can define the 'Either' type:

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

Instead of relying on a 'Switch' method, this approach leverages the 'is_left' and 'is_right' methods to determine the type of the contained value. The 'find_left' and 'find_right' methods then provide an 'Optional' value for the corresponding type.

Implementation Details and Usage

Within the 'Left' and 'Right' concrete types implementing 'Either,' we manage the actual payload and provide the necessary boolean flags. The 'left' and 'right' functions act as constructors, simplifying instantiation.

A sample usage demonstrates how to work with the 'Either' type:

func main() {
    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)
        } else {
            fmt.Fprintln(os.Stderr, err)
        }
    }
}

Conclusion

While Go's generic limitations pose challenges, it's possible to work around them and create powerful abstractions like the 'Either' type. By leveraging 'Optional' and implementing a custom interface with helper methods, developers can gain the flexibility and expressiveness they seek in their Go code.

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