Elm-lang: Basics of Json parsing

First import Json.Decode exposing (..). I'm on 4.0.5 of Elm's core library.

int is a function parses an Int value. There are similar functions for other json types, float for example. These are examples of Decoder a types.

You can have more advanced decoders that work upon the above simple decoders.

  • (:=) takes a string and a decoder type itself. It gets a field, named in the first paramtere, in a json object and applies the decoder on it, to extract that type.
  • list takes in a decoder itself and looks for a list of items confirming the the decoder type.

Decoders that take decoders allow for embedding: list ("thing" := int) says we should find a list of objects, and in those objects there should be a "thing" field which should be an integer.

More advanced decoders are ones that work with objects with multiple fields. object2 takes in two decoders. It also takes in a function that takes the two decoder values and returns something that combines them.

What's an example of "a function that takes the two decoder values and returns something"? If you have type alias Hiya = {field1: String, field2: String} it works as Hiya "one" "two" when used as a function.

So object2 Hiya ("field1":=string) ("field2":= string) would decode two string fields and return Hiya.

So we're both have simple and complex decoders now. What can we do with them?

decodeString will take a decoder such as the ones above and returns a Result String a. This returns either an error as a string or the value: a string value in the case of decodeString.

For example, given the type type alias Decoded = {id:Int, company:String} and the json:

    "companies": [
            "id": 2,
            "name": "Thing"
            "id": 1,
            "name": "Another Thing"

Then decodeString ("companies" := list (object2 Decoded ("id":=int) ("name":=string))) theJson would return you these Elm values wrapped in a Result: Ok ([{ id = 2, company = "Thing" },{ id = 1, company = "Another Thing" }])

But Http.get is probably the main use of this. It takes a Json.Decoder and decodes the Json values and returns Elm values according to the decoder passed.


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