JsonStuff - Elm native module to auto-encode/decode JSON

Api.elm

port module Api exposing (..)

import JsonStuff exposing (ToJson, FromJson, getEncoder, getDecoder)
import Http

helpPostJson : String -> ToJson a msg -> FromJson b msg -> a -> Http.Request b
helpPostJson url toJson fromJson o =
    Http.request
        { method = "POST"
        , headers = []
        , url = url
        , body = getEncoder toJson o |> Http.stringBody "application/json"
        , expect = getDecoder fromJson |> Http.expectJson
        , timeout = Nothing
        , withCredentials = False
        }


-- stating the obvious: types and msgs here for example only
-- they should probably not be in this file
type alias SomeType =
    { anInt : Int
    , aString : String
    }
    

type Msg
    = SomeTypeEchoed (Result Http.Error SomeType)


-- must declare ports so that the compiler generates encoders/decoders
-- outgoing generates encoder, incoming generates decoder
port someTypeOut : SomeType -> Cmd msg
port someTypeIn : (SomeType -> msg) -> Sub msg


echoSomeType : SomeType -> Cmd Msg
echoSomeType x =
    helpPostJson "http://localhost/echo" someTypeOut someTypeIn x
        |> Http.send SomeTypeEchoed

example.elm-package.json

{
    "version": "0.0.1",
    "summary": "Example project",
    "repository": "https://github.com/local/local.git",
    "license": "MIT",
    "source-directories": [
        "src/Elm"
    ],
    "exposed-modules": [],
    "dependencies": {
        "elm-lang/core": "5.0.0 <= v < 6.0.0",
        "elm-lang/html": "2.0.0 <= v < 3.0.0",
        "elm-lang/http": "1.0.0 <= v < 2.0.0",
        "elm-lang/navigation": "2.0.0 <= v < 3.0.0",
        "evancz/elm-markdown": "3.0.1 <= v < 4.0.0",
    },
    "native-modules": true,
    "elm-version": "0.18.0 <= v < 0.19.0"
}

JsonStuff.elm

module JsonStuff exposing (..)

import Native.JsonStuff
import Json.Decode exposing (Decoder)


type alias ToJson a msg =
    a -> Cmd msg


type alias FromJson a msg =
    (a -> msg) -> Sub msg


type alias Encode a =
    a -> String


getEncoder : ToJson a msg -> Encode a
getEncoder outPort =
    Native.JsonStuff.getEncoder outPort


getDecoder : FromJson a msg -> Decoder a
getDecoder inPort =
    Native.JsonStuff.getDecoder inPort

JsonStuff.js

// located at Native/JsonStuff.js
// must add "native-modules": true to elm-package.json
// first line must be changed to match author/project, format:
// var _[author]$[project]$Native_JsonStuff = function() {
var _local$local$Native_JsonStuff = function() {

  var getEncoder = function(portFn) {
    var portName = portFn(null).home;
    var toJsObject = _elm_lang$core$Native_Platform.effectManagers[portName].converter;
    var encoder = function(elmObject) {
      return JSON.stringify(toJsObject(elmObject));
    }
    return encoder;
  };

  var getDecoder = function(portFn) {
    var portName = portFn(null).home;
    var decoder = _elm_lang$core$Native_Platform.effectManagers[portName].converter;
    return decoder;
  };
  
  return {
    getEncoder : getEncoder,
    getDecoder : getDecoder
  };}
();

Thanks to @mpdairy for this repo which showed me how to do the native bits.