Fred and Betty exercise from Programming Elixir, Chapter 12 Write a program that spawns two processes, and then passes each a unique token (for example “fred” and “betty”). Have them send the tokens back.

fred_and_betty.exs

defmodule Echo do
  def echo(token, caller_pid) do
    IO.puts "Sending #{inspect token} to #{inspect caller_pid}"
    caller_pid <- { caller_pid, token }
  end
end

defmodule Receiver do
  def report(times) do
    Enum.map(1..times,
      fn(_) ->
        receive do
          { caller_pid, token } -> 
            caller_pid <- IO.puts "Received #{inspect token} back!"
          _ -> IO.puts "You shouldn't see this!"
        end
      end)
  end
end

# We need to do the above reassignment because
# the call the spawn changes the value of 'self'
me = self
spawn fn -> Echo.echo(:fred, me) end
spawn fn -> Echo.echo(:betty, me) end

# This won't work though (notice the 'self'):
# spawn fn -> Echo.echo(:fred, self) end
# spawn fn -> Echo.echo(:betty, self) end
# The value of self is the pid of the spawned process.

Receiver.report(2)