ConCache.update_existing

You're seeing just the function update_existing, go back to ConCache module for more information.
Link to this function

update_existing(cache_id, key, update_fun)

View Source

Specs

update_existing(t(), key(), update_fun()) ::
  :ok | {:error, :not_existing} | {:error, any()}

Updates the item only if it exists. Otherwise works just like update/3.