Debugging JSON

Published October 10, 2016 · 5 Minute Read · ∞ Permalink

A recurring theme in the Elm community (and this blog) is that JSON is kind of difficult to get started with. This makes sense, since it’s all about dealing with data that other people provide for us. It’s hard to debug what’s happening with JSON decoders since they’re often used in HTTP response decoders. So today we’re going to talk about some general advice: how do you even debug JSON?

The Pattern

Peter Damoc suggests one approach on the elm-discuss mailing list. His approach takes two steps:

  1. Debug the decoder with fake (but valid) data.
  2. Retrieve data from a live source

I think this makes a lot of sense, especially if we think about it as a problem of decoupling! If you don’t know how your server is going to act, it makes sense to take care of the responses you can expect before you tune your requests. That way, when you’re working with HTTP you don’t have to worry about unknown responses.

Our Goal

We’re going to show this off by deserializing some data from Typicode’s JSON Placeholder API We’re going to decode part of a user (/users/{id}) into the following model:

type alias User =
    { id : Int
    , name : String
    , username : String
    , email : String

Fake Data

Let’s get started with step 1: load some fake (but valid) data. I’ve retrieved this by running curl | pbcopy and pasting the result into my Elm module:

fake : String
fake =
  "id": 1,
  "name": "Leanne Graham",
  "username": "Bret",
  "email": "",
  "address": {
    "street": "Kulas Light",
    "suite": "Apt. 556",
    "city": "Gwenborough",
    "zipcode": "92998-3874",
    "geo": {
      "lat": "-37.3159",
      "lng": "81.1496"
  "phone": "1-770-736-8031 x56442",
  "website": "",
  "company": {
    "name": "Romaguera-Crona",
    "catchPhrase": "Multi-layered client-server neural-net",
    "bs": "harness real-time e-markets"

Cool! So what’s next? Well, we’ve got to write the JSON decoder and test it, then get those messages into our application. We know that we’ll soon want this to go in something that takes an Int for the user ID and returns a Cmd Msg. The function we’ll need to write ends up looking something like this:

loadUser : Int -> Cmd Msg
loadUser _ =
    Decode.decodeString userDecoder fake
        |> Task.fromResult
        |> Task.perform LoadingFailed UserLoaded

LoadingFailed and UserLoaded handle a failed and a successful result, respectively. We’re using something similar to Kris Jenkins’ RemoteData pattern here to make sure we handle our errors. This pattern helps make sure your application never gets in an undefined state, like Richard Feldman recommended in his elm-conf talk. Check out the GitHub Project or for more details how we’re defining these. There’s also a live demo! But now we can define our JSON decoder!

userDecoder : Decode.Decoder User
userDecoder =
        ("id" :=
        ("name" := Decode.string)
        ("username" := Decode.string)
        ("mail" := Decode.string)

Unfortunately, it looks like I’ve got an error! Since we’re displaying those, we get it right away:

UnexpectedPayload: Expecting an object with a field named `mail` but instead got [snip]

We can look through our fake response to see what happened. We’ll find that the field is indeed “email”, not “mail”. Other errors will give you similar helpful messages. As usual in Elm debugging, just follow the messages until your decoder works!

Real Data

Now that our decoder works (hooray!) we can just swap out our fake loadUser function for a real one. Mine ended up looking like this:

loadUser : Int -> Cmd Msg
loadUser i =
        url =
            "" ++ (toString i)
        Task.perform LoadingFailed UserLoaded (Http.get userDecoder url)

Since we’ve already made sure our JSON decoder works, we can now deal with everything else in isolation. Making our HTTP request is now a matter of communication, instead of both communication and deserialization.


Now you know! To debug your JSON decoders, separate your deserialization layer from your HTTP requests and you’ll have a much easier (and quicker) time of it!

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