Combine the mutex with the thing it is locking into a single struct. Often mutex elements of a struct are unnamed, since you only need to lock and unlock the whole thing

type SafeMap struct {
  sync.Mutex
  URLs map[string]bool
}

which you can then use like so:

url := "http://some_example.com"
fetched := SafeMap{URLs:map[string]bool{}}
fetched.Lock()
fetched.URLs[url] = true
fetched.Unlock()
 
fetched.URLs[url

Combine the mutex with the thing it is locking into a single struct. Often mutex elements of a struct are unnamed, since you only need to lock and unlock the whole thing

type SafeMap struct {
  sync.Mutex
  URLs map[string]bool
}

which you can then use like so:

url := "http://some_example.com"
fetched := SafeMap{URLs:map[string]bool{}}
fetched.Lock()
fetched.URLs[url] = true
fetched.Unlock()