Elixir Code Not receiving messages

Questions / Help
1

This is a piece of code for testing the receive block

defmodule ElixirBasics.Runner do
  def start do
    IO.puts("Started the process at #{inspect(self())}")
    receive_msg()
  end

  def receive_msg do
    receive do
      {:ok, msg} ->
        IO.puts("received this message #{msg}")

      _ ->
        IO.puts("received invalid msg type")
    end

    receive_msg()
  end
end

ElixirBasics.Runner.start()
Process.sleep(:infinity)

This is present inside a .ex file and running with the command mix run lib/runner.ex. Now I am trying to send messages to the spawned process using Process.send(pid of process, {:ok, “hello”}, [ ]). I got :ok when trying to run the send command but the messages weren’t handled by receive block. The IO statements weren’t getting logged into the console. Can anyone point out my mistake?

2

This works

iex(10)> pid = spawn(ElixirBasics.Runner, :start, [])
Started the process at #PID<0.118.0>
#PID<0.118.0>
iex(11)> Process.send(pid, {:ok, “hello”}, [])
received this message hello
:ok

or this if you want to run from same file -

defmodule ElixirBasics.Runner do
  def start do
    IO.puts("Started the process at #{inspect(self())}")
    receive_msg()
  end

  def receive_msg do
    receive do
      {:ok, msg} ->
        IO.puts("received this message #{msg}")

      _ ->
        IO.puts("received invalid msg type")
    end

    receive_msg()
  end
end

pid = spawn(ElixirBasics.Runner, :start, [])
Process.send(pid, {:ok, "hello"}, [])
4 Likes
3

FWIW, Process.send will not return an error if you try to send to a PID that doesn’t exist. There are only two not-:ok values it will return, and those only happen if you ask for them explicitly (:noconnect and :nosuspend)

2 Likes
4

Note however that send() will raise if the target is a registered name which is not found, which is one of the ugliest design mistakes in OTP (and there really aren’t many).

Strangely I just noticed that this is actually not documented for the Elixir Kernel.send/2. They do at least link to the Erlang docs, but I think it should be explicitly mentioned.

1 Like