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JSON Parsing in ELM
JSON parsing in a purely functional web language
Posted on Jan 14 2017 ~ 6 min read
#elm  #json  #functional programming 

Immutable State of the Art

There is a ongoing hype of JavaScript[1] in the developer community. Virtually everything is now compiled to JavaScript and delivered to the client. JavaScript is also becoming very popular for server side development[2], replacing Ruby and PHP as primary programming language.[9] Not only that, there’s also an ongoing serverless trend, moving most of the back-end code into the front-end part of the application.[3] This paradigm shift results in highly complex front-end applications, making it very complicated to write applications with plain old vanilla JavaScript. For this reason, the community started to put much effort in improving the JavaScript front-end development. A new JavaScript Standard ES6 (ECMAScript 2015) has been published. The Babel compiler, compiles that new standard to vanilla JavaScript, allowing it’s execution in older browsers. Frameworks and tools like react, webpack, karma, mocha, enzyme, gulp, eslint and many more to make the life as JavaScript developer as comfortable as possible.[5]

It takes many weeks to get familiar with the new toolchain, but it pays off in the long run. Following all current best practices leads to a setup with many different tools and a highly improved JavaScript syntax, that basically follows the approach of reactive functional programming. Why is that? Well, the major problem is mutable state.[8] Mutable state is not only a potential source of bugs, it is also one of the top reasons for unpredictable application behaviour and is in principle not compatible with parallelization.[6] So, why not experimenting with a new reactive functional programming language explicitly designed for front-end programming, that compiles to vanilla JavaScript.

Parse JSON in Elm 0.18

Elm is a static typed, reactive, pure functional programming language. It has a simplified Haskell syntax, making it much easier to use than Haskell. It is focused on front-end web development and therefore compiles to JavaScript. It has the builtin features of react and redux.[7]

Lets consider an JSON API in the following format:

    "name": "JavaScript",
    "year": "2012",
    "quarter": "2",
    "count": "118009"
    "name": "Ruby",
    "year": "2012",
    "quarter": "2",
    "count": "67773"
    "name": "Python",
    "year": "2012",
    "quarter": "2",
    "count": "42212"

Since Elm is strongly typed we have to create a type that matches the given JSON objects.

type alias ProgrammingLanguage = {
  name : String,
  year : Int,
  quarter : Int,
  count : Int

As you can see the attribute name is a string and year, quarter, count are integers. The JSON API provides integers encoded as strings, so it is necessary to do a type cast. Therefore we need to write a simple custom decoder, that converts a string to integer.

stringAsInt : Decoder Int
stringAsInt = string |> andThen (String.toInt >> fromResult)

Lets take a look at the type definition to see how this works.
andThen : (a -> Decoder b) -> Decoder a -> Decoder b
The function andThen takes two arguments a decoder a, in this case the standard string decoder and a type cast function from a to decoder b that converts the result of our string decoder to an new decoder of type b. This procedure is a simple chain, first decode as string and then decode as int by type conversion. If you know Haskell andThen is the equivalent of (=<<) :: Monad m=> (a -> m b) -> m a -> m b the monadic bind that sequentially compose two actions by passing any value produced by the first as an argument to the second. If you are not familiar with the pipe |> and function composition >> notation you can also write the function with as a lambda expression andThen (\n -> fromResult(String.toInt(n))) string. Now we are able to use that decoder to decode a single JSON object.

langDecoder : Decoder ProgrammingLanguage
langDecoder =
  map4 ProgrammingLanguage
    (field "name" string)
    (field "year" stringAsInt)
    (field "quarter" stringAsInt)
    (field "count" stringAsInt)

Since our API is a list of programming language objects we need to parse the whole list. Json.Decode.list : Decoder a -> Decoder (List a)

langListDecoder : Decoder (List ProgrammingLanguage)
langListDecoder = Json.Decode.list langDecoder

Finally you are able to use the decoder to parse JSON from a given URL Http.get url langListDecoder

getProgrammingLanguages : Cmd Msg
getProgrammingLanguages =
  let url = "gh-star-event.json"
  in Http.send NewLang (Http.get url langListDecoder)

See for a full working API parsing example.


There’s a new command line tool and website that allows for automatic conversion of JSON to elm code.

This project allows you to automate the creation of:

  • type aliases from JSON data
  • decoders from type aliases and some union types
  • encoders from type aliases and some union types