worker.ex

defmodule Metex.Worker do
    
    def loop do
        receive do
            {sender_pid, location} ->
                send(sender_pid, {:ok, temperature_of(location)})
            _ ->
                send(sender_pid, "Unknown message")
        end
        loop
    end
    
    def temperature_of(location) do
        result = url_for(location) |> HTTPoison.get |> parse_response
        case result do
            {:ok, temp} ->
                "#{location}: #{temp} C"
            :error ->
                "#{location} not found"
        end
    end

    defp url_for(location) do
        "http://api.openweathermap.org/data/2.5/weather?q=#{location}"
    end

    defp parse_response({:ok, %HTTPoison.Response{body: body, status_code: 200}}) do
        body |> JSON.decode! |> compute_temperature
    end

    defp parse_response(_) do
        :error
    end

    defp compute_temperature(json) do
        try do
            temp = (json["main"]["temp"] - 273.15) |> Float.round(1)
            {:ok, temp}
        rescue
            _ -> :error
        end
    end
end

Line 8 is what throws the error. Nothing is bound to sender_pid in that pattern.

iex -S mix
Erlang/OTP 17 [erts-6.2] [source-aaaefb3] [64-bit] [smp:8:8] [async-threads:10] [hipe] [kernel-poll:false]

== Compilation error on file lib/worker.ex ==
** (CompileError) lib/worker.ex:8: function sender_pid/0 undefined
(stdlib) lists.erl:1336: :lists.foreach/2
(stdlib) erl_eval.erl:657: :erl_eval.do_apply/6

😱 Second case statement is wrong. My bad!

You could probably replace

send(sender_pid, "Unknown message")

with

IO.puts "Unknown message")

Thanks for pointing it out!