Better debugging with IO.inspect and labels

Introduction#

IO.inspect is very useful when you try to debug your chains of method calling. It can get messy though if you use it too often.

Since Elixir 1.4.0 the label option of IO.inspect can help

Remarks#

Only works with Elixir 1.4+, but I can’t tag that yet.

Without labels

url
  |> IO.inspect
  |> HTTPoison.get!
  |> IO.inspect
  |> Map.get(:body)
  |> IO.inspect
  |> Poison.decode!
  |> IO.inspect

This will result in a lot of output with no context:

"https://jsonplaceholder.typicode.com/posts/1"
%HTTPoison.Response{body: "{\n  \"userId\": 1,\n  \"id\": 1,\n  \"title\": \"sunt aut facere repellat provident occaecati excepturi optio reprehenderit\",\n  \"body\": \"quia et suscipit\\nsuscipit recusandae consequuntur expedita et cum\\nreprehenderit molestiae ut ut quas totam\\nnostrum rerum est autem sunt rem eveniet architecto\"\n}",
 headers: [{"Date", "Thu, 05 Jan 2017 14:29:59 GMT"},
  {"Content-Type", "application/json; charset=utf-8"},
  {"Content-Length", "292"}, {"Connection", "keep-alive"},
  {"Set-Cookie",
   "__cfduid=d56d1be0a544fcbdbb262fee9477600c51483626599; expires=Fri, 05-Jan-18 14:29:59 GMT; path=/; domain=.typicode.com; HttpOnly"},
  {"X-Powered-By", "Express"}, {"Vary", "Origin, Accept-Encoding"},
  {"Access-Control-Allow-Credentials", "true"},
  {"Cache-Control", "public, max-age=14400"}, {"Pragma", "no-cache"},
  {"Expires", "Thu, 05 Jan 2017 18:29:59 GMT"},
  {"X-Content-Type-Options", "nosniff"},
  {"Etag", "W/\"124-yv65LoT2uMHrpn06wNpAcQ\""}, {"Via", "1.1 vegur"},
  {"CF-Cache-Status", "HIT"}, {"Server", "cloudflare-nginx"},
  {"CF-RAY", "31c7a025e94e2d41-TXL"}], status_code: 200}
"{\n  \"userId\": 1,\n  \"id\": 1,\n  \"title\": \"sunt aut facere repellat provident occaecati excepturi optio reprehenderit\",\n  \"body\": \"quia et suscipit\\nsuscipit recusandae consequuntur expedita et cum\\nreprehenderit molestiae ut ut quas totam\\nnostrum rerum est autem sunt rem eveniet architecto\"\n}"
%{"body" => "quia et suscipit\nsuscipit recusandae consequuntur expedita et cum\nreprehenderit molestiae ut ut quas totam\nnostrum rerum est autem sunt rem eveniet architecto",
  "id" => 1,
  "title" => "sunt aut facere repellat provident occaecati excepturi optio reprehenderit",
  "userId" => 1}

With labels

using the label option to add context can help a lot:

url
  |> IO.inspect(label: "url")
  |> HTTPoison.get!
  |> IO.inspect(label: "raw http resonse")
  |> Map.get(:body)
  |> IO.inspect(label: "raw body")
  |> Poison.decode!
  |> IO.inspect(label: "parsed body")

url: "https://jsonplaceholder.typicode.com/posts/1"
raw http resonse: %HTTPoison.Response{body: "{\n  \"userId\": 1,\n  \"id\": 1,\n  \"title\": \"sunt aut facere repellat provident occaecati excepturi optio reprehenderit\",\n  \"body\": \"quia et suscipit\\nsuscipit recusandae consequuntur expedita et cum\\nreprehenderit molestiae ut ut quas totam\\nnostrum rerum est autem sunt rem eveniet architecto\"\n}",
 headers: [{"Date", "Thu, 05 Jan 2017 14:33:06 GMT"},
  {"Content-Type", "application/json; charset=utf-8"},
  {"Content-Length", "292"}, {"Connection", "keep-alive"},
  {"Set-Cookie",
   "__cfduid=d22d817e48828169296605d27270af7e81483626786; expires=Fri, 05-Jan-18 14:33:06 GMT; path=/; domain=.typicode.com; HttpOnly"},
  {"X-Powered-By", "Express"}, {"Vary", "Origin, Accept-Encoding"},
  {"Access-Control-Allow-Credentials", "true"},
  {"Cache-Control", "public, max-age=14400"}, {"Pragma", "no-cache"},
  {"Expires", "Thu, 05 Jan 2017 18:33:06 GMT"},
  {"X-Content-Type-Options", "nosniff"},
  {"Etag", "W/\"124-yv65LoT2uMHrpn06wNpAcQ\""}, {"Via", "1.1 vegur"},
  {"CF-Cache-Status", "HIT"}, {"Server", "cloudflare-nginx"},
  {"CF-RAY", "31c7a4b8ae042d77-TXL"}], status_code: 200}
raw body: "{\n  \"userId\": 1,\n  \"id\": 1,\n  \"title\": \"sunt aut facere repellat provident occaecati excepturi optio reprehenderit\",\n  \"body\": \"quia et suscipit\\nsuscipit recusandae consequuntur expedita et cum\\nreprehenderit molestiae ut ut quas totam\\nnostrum rerum est autem sunt rem eveniet architecto\"\n}"
parsed body: %{"body" => "quia et suscipit\nsuscipit recusandae consequuntur expedita et cum\nreprehenderit molestiae ut ut quas totam\nnostrum rerum est autem sunt rem eveniet architecto",
  "id" => 1,
  "title" => "sunt aut facere repellat provident occaecati excepturi optio reprehenderit",
  "userId" => 1}