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Could someone explained what the Key in the CoroutineContextHandler is used for? I can’t figure it out from the <https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-coroutine-exception-handler/index.html|documentation>

It's mostly an implementation détail to retrieve the CoroutineExceptionHandler in a CoroutineContext. See  <https://link.medium.com/sJGX3mKwamb|https://link.medium.com/sJGX3mKwamb> for more details

See the CoroutineContext as a (typed) map. You can put and get an Element by providing it a Key. Putting another Element with the same key will replace the current Element, if there is one.

By convention (not enforced by the language or the compiler), a subtype of an Element should have companion object that is a subtype of Key.

```class MyElement(...) : Element {
  ...
  fun hello() { ... }
  companion object : Key&lt;MyElement&gt;
}
...
...
coroutineContext[MyElement] = MyElement(...)
...
coroutineContext[MyElement].hello()
                     ^-- key is companion object```