Swift "for" inside of ( ) what is this..? : <pre><code>func getIndex(for priority: Task.Priority) -&gt; Int
{
    prioritizedTasks.firstIndex { $0.priority == priority }!
}
</code></pre>

<p>what is "for" inside of ()? It is for loop...? what is $0 ? I tried to find them in apple document but I don't know the operator name for them...</p>
