How Can I Access Map Keys in Go Regardless of Value Type?

Working with Maps with Distinct Value Types

In Go, maps are versatile data structures that map keys to specific values. However, situations may arise where we desire to utilize the keys of multiple maps, even if their value types differ. Consider the following code snippet:

func main() {
    mapOne := map[string]int
    mapTwo := map[string]double
    mapThree := map[string]SomeStruct

    useKeys(mapOne)
}

func useKeys(m map[string]interface{}) {
    // something with keys here
}

In this example, we have three maps: mapOne holds integers, mapTwo stores double values, and mapThree contains instances of SomeStruct. Our goal is to create a function useKeys that accepts a map with string keys and an arbitrary value type denoted by interface{}. However, this design doesn't function as expected.

The Challenge of Covariance

In Go, maps and slices are generic structures, but they lack covariance. This implies that while you can assign a map[string]int to a variable of type map[string]interface{}, you cannot safely modify the values of the assigned map without causing errors.

A Less Elegant, But Functional Solution

If we solely need to obtain the keys of a map regardless of its value type, we can resort to reflection:

func useKeys(m interface{}) {
    v := reflect.ValueOf(m)
    if v.Kind() != reflect.Map {
        fmt.Println("not a map!")
        return
    }

    keys := v.MapKeys()
    fmt.Println(keys)
}

This solution isn't as elegant as we would like, but it effectively retrieves and prints the keys of any map passed to it, irrespective of the contained value types.

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