ELM : decoding JSON

23 Jun 2018

In ELM understanding decoding JSON will take a bit of time. We can define a Decoder as a spec on how to perform the translation from JSON to ELM types.

In this post we will try to understand the how to decode a JSON string to ELM types. For limiting the scope we Will cover decoding Simple Object in this post and the others complex ones like decoding List of Objects, decoding Union Types, decoding JSON with optional keys will cover in other posts.

We will use Json.Decode package from ELM core to do the decoding.

json card

photo credit: superfluity JSON Card – Front via photopin (license)

simple object

Lets get started with defining a simple json string for easy decoding. We will also define an ELM type to which this json should decode into.

import Json.Decode

type alias Obj = 
    { a: Int, b: String }

json = 
    """
    {"a": 10, "b": "abc"}
    """

Next lets define a custom decoder function called decodeObjValue, which is composed from the different methods available in Json.Decode library. This will defined as per the structure of our json and Obj.

decodeObjValue : Json.Decode.Decoder Obj
decodeObjValue =
    Json.Decode.map2 Obj
        (Json.Decode.field "a" Json.Decode.int)
        (Json.Decode.field "b" Json.Decode.string)

Since we have only 2 fields in the JSON string we can use Json.Decode.map2 and pass Obj and functions to decode each field. Our first field a is of type Int. So we can use (Json.Decode.field "a" Json.Decode.int) and for b we use (Json.Decode.field "b" Json.Decode.string) since we are expecing b to be of type String

Note: It is extremely important to keep the order of the fields. The decoding will fail if we try to decode the field b first.

In Elm, all type alias’s are shorthand functions In our case Obj is (\a b -> Obj a b) This is the reason why we should keep the order of field. If we try to decode b first the Obj will receive a String as first param and it can’t accept it.

Now we defined or agreed on how to decode the incoming json. Next we need to run this decoder. Since our input is String we will use Json.Decode.decodeString to parse the given string and run our custom decoder.

decodeObj : String -> Obj
decodeObj json =
    case Json.Decode.decodeString decodeObjValue json of
        Ok obj ->
            obj
        Err _ ->
            {a = -1, b = "Error"}

Json.Decode.decodeString will accept the Decoder as first argument and json string which need to decoded as second. It returns the type Result which we need use it with case statement to get the exact result.

You can see the decode in action on ellie-app

Versions of Language/packages used in this post.

| Library/Language | Version |
| ---------------- |---------|
|      ELM         |  0.18.0 |
|      core        |  5.1.1  |
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