How Do Go Generics Handle Unions and Their Limitations in Type Constraints and Containers?

Go Generics - Unions and Their Limitations

In Go, unions serve two primary purposes:

1. Specifying type sets in interface constraints: Unions allow you to restrict a generic type parameter to a set of specific types.
2. Including approximate elements: Using the ~ symbol in a union indicates that other compatible types may be included.

However, it's crucial to note that unions can only be used in interface constraints. Trying to use them as regular types, like in tt := []testDifference[intOrString], is not supported.

Additionally, generic containers, such as slices, cannot hold items of different types. While your test slice includes both testDifference[int] and testDifference[string], these concrete instantiations are different types.

To solve this, you can either use a separate slice for each type (ttInts and ttStrs) or store all items as interfaces ([]interface{}).

Operations on union constraints are limited to those supported by all types in the type set. In the case of int | string, this includes variable declaration, conversions, comparisons, ordering, and the operator.

Example:

type intOrString interface {
    int | string
}

func beforeIntOrString[T intOrString](a, b T) bool {
    return a < b
}

func sumIntOrString[T intOrString](a, b T) T {
    return a + b
}

func main() {
    fmt.Println(beforeIntOrString("foo", "bar")) // false
    fmt.Println(beforeIntOrString(4, 5)) // true

    fmt.Println(sumIntOrString("foo", "bar")) // foobar
    fmt.Println(sumIntOrString(10, 5)) // 15
}

In this example, beforeSendIntOrString and sumIntOrString are generic functions that work with int and string values, respectively.

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