Here are a few title options, each highlighting a different aspect of the article: Focusing on the problem: * How to Set Fields in Different Firebase Message Structs with Generics in Go 1.18? * Gene

Generic Function for Struct Members from External Packages

Consider the goal of creating a single function for adding specific fields to different Firebase message structs, like Message and MulticastMessage, which share common fields of similar types. Initially, an attempt to define a generic function highPriority using a type constraint as follows yielded an error:

<code class="go">type firebaseMessage interface {
    *messaging.Message | *messaging.MulticastMessage
}

func highPriority[T firebaseMessage](message T) T {
    message.Android = &messaging.AndroidConfig{...}
    return message
}</code>

Limitations of Go 1.18

In Go 1.18, accessing common fields or methods of type parameters is not supported. Therefore, this approach fails.

Solution 1: Type Switch

For a limited number of types in the union, a type switch can be utilized:

<code class="go">func highPriority[T firebaseMessage](message T) T {
    switch m := any(message).(type) {
    case *messaging.Message:
        setConfig(m.Android)
    case *messaging.MulticastMessage:
        setConfig(m.Android)
    }
    return message
}</code>

Solution 2: Wrapper with Method

Another approach involves defining a wrapper type that implements a common method to set the desired config:

<code class="go">type wrappedMessage interface {
    *MessageWrapper | *MultiCastMessageWrapper
    SetConfig(c foo.Config)
}

// ...

func highPriority[T wrappedMessage](message T) T {
    message.SetConfig(messaging.Android{"some-value"})
    return message
}</code>

Solution 3: Reflection

For scenarios with numerous structs, reflection can be employed:

<code class="go">func highPriority[T firebaseMessage](message T) T {
    cfg := &messaging.Android{}
    reflect.ValueOf(message).Elem().FieldByName("Android").Set(reflect.ValueOf(cfg))
    return message
}</code>

Additional Notes:

• For more information on type parameter limitations, refer to [How can I define a struct field in my interface as a type constraint?](https://go.dev/play/p/9iG0eSep6Qo).
• For details on implementing common methods for types in a union constraint, visit [In Go generics, how to use a common method for types in a union constraint?](https://go.dev/play/p/JUHp9Fu27Yt).

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