Interfacing with the erlang xmerl library to parse xml data. Using xml files from weather.gov for sample data (http://w1.weather.gov/xml/current_obs/)

weather_parser.ex

defmodule WeatherParser do
  require Record
  Record.defrecord :xmlElement, Record.extract(:xmlElement, from_lib: "xmerl/include/xmerl.hrl")
  Record.defrecord :xmlText, Record.extract(:xmlText, from_lib: "xmerl/include/xmerl.hrl")

  def parse(xml, attrs) do
    xml
    |> :binary.bin_to_list
    |> :xmerl_scan.string
    |> extract_values(attrs, [])
  end

  def extract_values(xml, [], res), do: Enum.reverse(res)
  
  def extract_values({xml, _rest}, list, res), do: extract_values(xml, list, res)
  
  def extract_values(xml, [h | tail], res) do
    val = extract_element(xml, h)
          |> extract_content
          |> extract_value

    extract_values(xml, tail, [val | res])
  end

  def extract_element(xml, name), do: :xmerl_xpath.string('/current_observation/#{name}', xml)
  
  def extract_content([element]), do: xmlElement(element, :content)
  
  def extract_value([content_element]), do: xmlText(content_element, :value)
end

defmodule WeatherParserTest do
  use ExUnit.Case

  test "returns the specified attributes" do
    data = WeatherParser.parse(sample_xml, [:location, :temp_f, :temp_c])
    assert data == ['Unknown Station', '39.2', '4.0']
  end

  def sample_xml do
  """
  <?xml version="1.0" encoding="ISO-8859-1"?>
  <?xml-stylesheet href="latest_ob.xsl" type="text/xsl"?>
  <current_observation version="1.0"
     xmlns:xsd="http://www.w3.org/2001/XMLSchema"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:noNamespaceSchemaLocation="http://www.weather.gov/view/current_observation.xsd">
    <credit>NOAA's National Weather Service</credit>
    <credit_URL>http://weather.gov/</credit_URL>
    <image>
      <url>http://weather.gov/images/xml_logo.gif</url>
      <title>NOAA's National Weather Service</title>
      <link>http://weather.gov</link>
    </image>
    <suggested_pickup>15 minutes after the hour</suggested_pickup>
    <suggested_pickup_period>60</suggested_pickup_period>
    <location>Unknown Station</location>
    <station_id>ALIA2</station_id>
    <observation_time>Last Updated on Feb 13 2016, 2:30 pm AST</observation_time>
          <observation_time_rfc822>Sat, 13 Feb 2016 14:30:00 -0400</observation_time_rfc822>
    <temperature_string>39.2 F (4.0 C)</temperature_string>
    <temp_f>39.2</temp_f>
    <temp_c>4.0</temp_c>
    <water_temp_f>40.3</water_temp_f>
    <water_temp_c>4.6</water_temp_c>
    <wind_string>Northeast at 9.2 MPH (7.97 KT)</wind_string>
    <wind_dir>Northeast</wind_dir>
    <wind_degrees>60</wind_degrees>
    <wind_mph>9.2</wind_mph>
    <wind_gust_mph>0.0</wind_gust_mph>
    <wind_kt>7.97</wind_kt>
    <pressure_string>988.5 mb</pressure_string>
    <pressure_mb>988.5</pressure_mb>
    <windchill_string>33 F (1 C)</windchill_string>
          <windchill_f>33</windchill_f>
          <windchill_c>1</windchill_c>
    <mean_wave_dir>Northeast</mean_wave_dir>
    <mean_wave_degrees></mean_wave_degrees>
    <disclaimer_url>http://weather.gov/disclaimer.html</disclaimer_url>
    <copyright_url>http://weather.gov/disclaimer.html</copyright_url>
    <privacy_policy_url>http://weather.gov/notice.html</privacy_policy_url>
  </current_observation>
  """
  end
end