How Can I Effectively Unit Test Go Generics Functions with Mixed Types in a Union Constraint?

Go Generics - Unions and Unit Testing with Mixed Types

Understanding Unions

Unions in Go generics represent a set of types that can be used to constrain a generic type parameter. They are solely employed in interface constraints, indicating that a generic type parameter T can only accept types included in the union.

Using Unions in Unit Testing

When writing unit tests for generic functions, you may encounter the challenge of testing with different types. In your case, you aimed to test your Difference function with both ints and strings within the same table test.

Why intOrString Interface Fails

Your attempt to create an intOrString interface constraint for testing encountered an error because intOrString is not a valid type. It is a constraint that restricts the generic type parameter T to either int or string. You cannot directly instantiate a type using a type constraint.

Solution: Using a Generic Container

Instead of using an intOrString constraint, you should declare your test input and output slices as generic types with the intOrString constraint:

type testDifferenceInput[T intOrString] [][]T
type testDifferenceOutput[T intOrString] []T

However, this approach won't solve your problem entirely. A generic container cannot hold items of different types. In your test slice, you have two distinct types: testDifference[int] and testDifference[string].

Separating Slices by Type

To accommodate different types in your unit test, you can separate the slices by type:

ttInts := []testDifference[int]{
    // ... test cases for ints
}
ttStrs := []testDifference[string]{
    // ... test cases for strings
}

Operations on Union Constraints

Union constraints limit the operations that can be performed on the generic type parameter. In the case of int | string, only operations applicable to both types are permitted, such as comparisons, ordering, and addition (only for strings).

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