Breaking Down Decoders From the Bottom Up

Published September 5, 2017 · 5 Minute Read · ∞ Permalink

Last week, we covered how to break down decoders by starting from the innermost, or topmost, part. But what if you’re having trouble breaking things down from the top? (Or you’re dealing with a really complex JSON schema?)

This week, let’s look at it from a different perspective: the outermost structure in (or the bottom up!)

Remember, we’re dealing with this JSON:

{
  "Site1": {
    "PC1": {
      "ip": "x.x.x.x",
      "version": "3"
    },
    "PC2": {
      "ip": "x.x.x.x",
      "version": "3"
    }
  },
  "Site2": {
    "PC1": {
      "ip": "x.x.x.x",
      "version": "3"
    },
    "PC2": {
      "ip": "x.x.x.x",
      "version": "3"
    }
  }
}

When we looked at it before, we started from the innermost value. I told you then that it’s a good approach because the structure doesn’t change too much. But sometimes it’s way too difficult to see the whole structure at once. Then, you want to let the compiler guide your steps. In that case, we can use Json.Decode.value!

The documentation for value describes it likes this:

Do not do anything with a JSON value, just bring it into Elm as a Value. This can be useful if you have particularly crazy data that you would like to deal with later. Or if you are going to send it out a port and do not care about its structure.

Hey! That fits us! We want to be able to decode our first level, and not worry about the rest. So let’s do that! (and as an aside: I’m not a huge fan of calling things “crazy” so we’re going to be referring to them as “unusual” or “weird” here.)

value is a Decoder Value, where Value is an opaque type (meaning we can’t see into it, like a frosted glass window.) We already know that we have a Dict with site names, so let’s tell Elm that.

sites : Decoder (Dict String Value)
sites =
    dict value

This says “we want the keys as strings, but don’t particularly care about the values”. When we try to decode, we get exactly what we asked for:

decodeString sites ourSitesBlob 
  == Ok (Dict.fromList 
       [ ("Site1", { PC1 = { ip = "x.x.x.x", version = "3" }, PC2 = { ip = "x.x.x.x", version = "3" } })
       , ("Site2", { PC1 = { ip = "x.x.x.x", version = "3" }, PC2 = { ip = "x.x.x.x", version = "3" } })
       ])

Now, you might look at this and go “well, hey, why can’t I use those things I can see in Value?” But remember that we haven’t told Elm what types they are! That means the compiler can’t check that we’re using these in a way that makes sense. To make a reliable app, we need to attach that information using decoders. Yet there is a benefit to being able to see our remaining structure: it gives us our next step!

We already knew that we had another mapping of names to metadata, but now we can see that. Using this information, let’s add another layer!

sites : Decoder (Dict String (Dict String Value))
sites =
    dict (dict value)

When we decode it, we get:

decodeString sites ourSitesBlob 
  == Ok (Dict.fromList 
       [ ("Site1"
         , Dict.fromList 
           [ ( "PC1", { ip = "x.x.x.x", version = "3" } )
           , ( "PC2", { ip = "x.x.x.x", version = "3" } )
           ]
         )
       , ( "Site2"
         , Dict.fromList 
           [ ( "PC1", { ip = "x.x.x.x", version = "3" } )
           , ( "PC2", { ip = "x.x.x.x", version = "3" } ) 
           ]
         )
       ])

Our unknown Values have shrunk down, and we can see that we’re almost done! Now we need one last step: our machine decoder from before:

type alias Machine =
    { ip : String
    , version : String
    }


machine : Decoder Machine
machine =
    map2 Machine
        (field "ip" string)
        (field "version" string)

With that, we can define our final decoder without using value:

sites : Decoder (Dict String (Dict String Machine))
sites =
    dict (dict machine)

And we’re done!

Now It’s Your Turn!

Now you want to apply this to your own projects, right? To break down a JSON object from the bottom up, you’ll need to:

1. Decode the information one level at a time, filling in value for levels you haven’t reached yet.
2. Use the REPL or tests to verify that what you wrote worked.
3. Refine any remaining values with steps 1 and 2 until you’re done!

If you get stuck, you might be trying to tackle too much at once! Break down the part you’re trying to do into the smallest possible unit and solve that, and build back up from there.

You’ll know you’re making progress if, after each step, your Value objects get smaller.

And remember, think with building blocks, not huge blobs!