How Can I Effectively Use Go Generics Unions in Type Constraints for Testing?

Go Generics - Type Constraints and Unions Explained

In Go generics, unions play a specific role as part of interface constraints. Let's unpack what this means and how it relates to your question about testing with different types.

What are Unions?

In generic type constraints, unions define a set of types that a type parameter must match. For instance:

type intOrString interface {
    int | string
}

This constraint ensures that any generic type parameter T must be either an int or a string.

Why Can't Unions Be Used as Types?

An interface constraint with a union is not a regular interface type. This distinction is intentional in Go's generic design.

Allowed Operations on Union Constraints

Functions using a type parameter with a union constraint can only perform operations permitted by every member of the union set. This includes:

• Variable declarations
• Conversions
• Comparisons
• Ordering
• The operator

Applying to Your Question

Your original approach used intOrString as a type, which isn't allowed. To use a union constraint correctly, modify your code as follows:

type testDifferenceInput[T intOrString] [][]T
type testDifferenceOutput[T intOrString] []T
type testDifference[T intOrString] struct {
    input testDifferenceInput[T]
    output testDifferenceOutput[T]
}

func TestDifference(t *testing.T) {
    var ttInts []testDifference[int]
    var ttStrings []testDifference[string]

    // Populate ttInts and ttStrings with test cases

    for _, tt := range append(ttInts, ttStrings) {
        // Execute the test case
    }
}

By separating the test cases based on the type parameter, you can work around the limitation that a generic container can't hold items of different types.

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