Print the key/value types of a Golang map

Question

I am trying to print the type of a map, eg: map[int]string

func handleMap(m reflect.Value) string {

    keys := m.MapKeys()
    n := len(keys)

    keyType := reflect.ValueOf(keys).Type().Elem().String()
    valType := m.Type().Elem().String()

    return fmt.Sprintf("map[%s]%s>", keyType, valType)
}

so if I do:

log.Println(handleMap(make(map[int]string)))

I want to get "map[int]string"

but I can't figure out the right calls to make.

Answer 1

Try not to use reflect. But if you must use reflect:

• A reflect.Value value has a Type() function, which returns a reflect.Type value.
• If that type's Kind() is reflect.Map, that reflect.Value is a value of type map[T1]T2 for some types T1 and T2, where T1 is the key type and T2 is the element type.

Therefore, when using reflect, we can pull apart the pieces like this:

func show(m reflect.Value) {
    t := m.Type()
    if t.Kind() != reflect.Map {
        panic("not a map")
    }
    kt := t.Key()
    et := t.Elem()
    fmt.Printf("m = map from %s to %s\n", kt, et)
}

See a more complete example on the Go Playground. (Note that both maps are actually nil, so there are no keys and values to enumerate.)