It’s a little weird, but my personal style is to put the @impl on a standalone function head (when defining multiple clauses for the function).

def Worker do
  use GenServer

  @impl GenServer
  def handle_call(request, from, state)

  def handle_call(:foo, from, state) do
    # ..
  end
  
  def handle_call(:bar, from, state) do
    # ..
  end
end

I guess in my mind it suggests I’m about to define multiple clauses for the same function as opposed to recognizing multiple definitions happen to be for the same function.

Only the first one is necessary.

Hello, I just add @impl true just to the first function. The benefit of @impl is to get warnings from the compiler and this will be reached with one annotation.

It’s worth noting that another benefit of @impl is to signal to the programmer that something is, indeed, a callback implementation. This isn’t as apparent in something as ubiquitous as a GenServer, but when reading a module implementing a less well-known or custom behaviour, this is really nice-to-have info.