Elixir's parallel map algorithm

Question

I am currently working through "Programming Elixir" book, and just found this example of "map" implemented in a parallel manner:

defmodule Parallel do
  def pmap(collection, fun) do
    me = self
    collection
    |> Enum.map(fn (elem) ->
      spawn_link fn -> (send me, { self, fun.(elem) }) end end)
    |> Enum.map(fn (pid) ->
      receive do { ^pid, result } -> result end
    end)
  end
end

The comment to the code says: "Note how it uses ^pid in the receive block to get the result for each PID in turn. Without this we’d get back the results in random order.". Can someone clarify this?

Answer 1

The ^ is called the pin operator. It matches with the previous value of the variable instead of rebinding it to the new value. So:

receive do
  {pid, result} -> result
end

receives any message of the form {whatever, whatever} and returns the second element from that tuple. But

receive do
  {^pid, result} -> result
end

receives any message that's a tuple and the first element is some exact value that's stored in pid.