Learning Elixir: A shitty program that does things with data

README.markdown

Crapitals

Filip Learns Elixir, Day 1

Basically, it's a thing which reads data from a CSV file, keeps it in memory using a stateful process CountryList, and lets you query said list. I tried using several of Elixir's features here such as processes, messaging, pattern-matching, and the pipe operator.

crapitals.csv

crapitals.ex

defmodule CountryList do
  def start_link(initial \\ []), do: Task.start_link(fn -> loop(initial) end)

  defp loop(list) do
    receive do
      {:get, caller} ->
        send caller, {:get, list}
        loop list
      {:get, caller, :by_name, name} ->
        send caller, {:get, :by_name, Enum.filter(list, &(&1.country == name))}
        loop list
      {:add, entry} ->
        loop [entry | list]
    end
  end
end

defmodule Util do
  defp make_list(binary) do
    String.split(binary, "\n")
    |> Enum.map(&String.split(String.strip(&1), ","))
    |> Enum.map(fn
      ([cn, cp, pop]) -> %{country: cn, capital: cp, population: elem(Integer.parse(pop), 0)}
      ([""]) -> nil
    end)
    |> Enum.reject(&is_nil/1)
  end

  @doc """
  Creates a CountryList process from file
  """
  def country_list_from_file (path \\ "crapitals.csv") do
    case File.read(path) do
      {:ok, content} -> CountryList.start_link make_list(content)
      {:error, posix} -> {:error, :file, posix}
    end
  end
end

defmodule Crapitals do
  def main() do
    {:ok, pid} = Util.country_list_from_file
    main :next, pid
  end

  defp main(:next, list) do
    case get_command() do
      {:error, :bad_command} ->
        IO.puts "Sorry, unrecognised command"
        main :next, list
      {:error, :bad_value, msg} ->
        IO.puts "Sorry, you entered a bad value. #{msg}"
        main :next, list
      {:by_name, name} ->
        send list, {:get, self(), :by_name, name}
        receive do
          {:get, :by_name, [data | _]} ->
            %{country: cn, capital: cp, population: pop} = data
            IO.puts "Found #{cn} (#{cp}) (#{pop} people)"
          {:get, :by_name, []} ->
            IO.puts "Country not found"
        end
        main :next, list
      :exit ->
        IO.puts "Bye then"
    end
  end

  defp get_command() do
    case String.split(IO.gets("> "), ~r/\s+/, trim: true) do
      ["exit"] -> :exit
      ["by_name", name] -> {:by_name, name}
      _ -> {:error, :bad_command}
    end
  end
end