Encode and decode JSON values

elm, json
elm-package install elm/json 1.1.3


JSON in Elm

This package helps you convert between Elm values and JSON values.

This package is usually used alongside elm-lang/http to talk to servers or ports to talk to JavaScript.


As described in Evan’s The Life of a File talk, it is best to build modules around types in Elm. That strategy is pretty helpful with JSON encoders and decoders, so let’s see how it goes in the following scenario:

Imagine you studied this causes of death table. Imagine you were shocked to find that in 2002, war accounted for 0.3% of global deaths whereas road traffic accidents accounted for 2.09% and diarrhea accounted for 3.15%. Or that there are big disparities between males and females for some causes, like fires and drowning.

Inspired by all these unexpected results, we decide to visualize this data in a nicer way. Well, we need some way to get the cause-of-death data from our server:

module Cause exposing (Cause, encode, decoder)

import Json.Decode as D
import Json.Encode as E


type alias Cause =
  { name : String
  , percent : Float
  , per100k : Float


encode : Cause -> E.Value
encode cause =
    [ ("name", E.string cause.name)
    , ("percent", E.float cause.percent)
    , ("per100k", E.float cause.per100k)


decoder : D.Decoder Cause
decoder =
  D.map3 Cause
    (D.field "name" D.string)
    (D.field "percent" D.float)
    (D.field "per100k" D.float)

Now in some other code we can use Cause.encode and Cause.decoder as building blocks. So if we want to decode a list of causes, saying Decode.list Cause.decoder will handle it!

Point is, the goal should be:

  1. Make small JSON decoders and encoders.
  2. Snap together these building blocks as needed.

So say you decide to make the name field more precise. Instead of a String, you want to use codes from the International Classification of Diseases recommended by the World Health Organization. These codes are used in a lot of mortality data sets! So it may make sense to make a separate IcdCode module with its own IcdCode.encode and IcdCode.decoder that ensure you are working with valid codes. From there, you can use them as building blocks in the Cause module!

The Future of JSON

I think it is a mistake to think that all data interchange will happen in JSON forever. People used to think similar things about XML, but these are both quite imperfect interchange formats. So in the long run, the goal is to head towards this vision of data interchange.