How to implement your own :timer.sleep in Elixir

Elixir has :timer.sleep(ms), a function which allows you to pause a process’s execution for an arbitrary number of milliseconds.

We can implement the same using messages and a timeout in the following ways.

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defmodule SleepTimeout do
  def sleep(milliseconds) do
    # use a receive block with no matching blocks and an after block
    # which times out after the input number of milliseconds
    receive do
    after
      milliseconds ->
        :ok
    end
  end
end

defmodule SleepMsg do
  def sleep(milliseconds) do
    # create a unique ref, so that we don't stop on any random `:timeout` message.
    ref = make_ref()
    # schedule a message to be sent to ourselves in the input number of milliseconds
    Process.send_after(self(), {:timeout, ref}, milliseconds)

    receive do
      # wait for our message to arrive
      {:timeout, ^ref} ->
        :ok
    end
  end
end

{microseconds, _} = :timer.tc(fn -> SleepTimeout.sleep(1000) end)
IO.puts("slept for #{round(microseconds / 1000)} microseconds")
# => slept for 1000 microseconds


{microseconds, _} = :timer.tc(fn -> SleepMsg.sleep(1000) end)
IO.puts("slept for #{round(microseconds / 1000)} microseconds")
# => slept for 1001 microseconds

You may ask, what is the frigging point of all of this when we have :timer.sleep? Well, there is no point. This was just a fun little exercise :D

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