Cookbooks
Elm
Chapter 10. Data Formats and Serialization
Working with JSON

Elm:
Working with JSON

How to:

Elm treats JSON handling with explicitness and safety, primarily using the Json.Decode and Json.Encode modules. To start working with JSON, you first need to define a decoder for your data type. Let’s assume we’re dealing with a simple user profile object.

Firstly, define your Elm type:

type alias UserProfile = 
    { id : Int
    , name : String
    , email : String
    }

Decoding JSON into Elm

To decode a JSON string into the UserProfile type, create a decoder:

import Json.Decode exposing (Decoder, int, string, field, map3)

userProfileDecoder : Decoder UserProfile
userProfileDecoder =
    map3 UserProfile
        (field "id" int)
        (field "name" string)
        (field "email" string)

To decode a JSON object:

import Json.Decode exposing (decodeString)

jsonString : String
jsonString = 
    """{"id": 1, "name": "John Doe", "email": "john@example.com"}"""

decoded : Result String UserProfile
decoded =
    decodeString userProfileDecoder jsonString

{- Sample Output:
Result.Ok { id = 1, name = "John Doe", email = "[email protected]" }
-}

Encoding Elm into JSON

To encode an Elm value back into JSON, leverage the Json.Encode module.

import Json.Encode exposing (object, int, string)

encodeUserProfile : UserProfile -> String
encodeUserProfile userProfile =
    object
        [ ("id", int userProfile.id)
        , ("name", string userProfile.name)
        , ("email", string userProfile.email)
        ]
        |> Json.Encode.encode 0

{- 
Usage:
encodeUserProfile { id = 1, name = "John Doe", email = "[email protected]" }

Sample Output:
"{"id":1,"name":"John Doe","email":"[email protected]"}"
-}

Third-Party Libraries

Elm packages like elm-json-decode-pipeline can simplify the creation of decoders using a pipeline style, which is especially handy for decoding complex objects.

First, add the library to your project:

elm install NoRedInk/elm-json-decode-pipeline

Then, you can simplify the decoder definition as follows:

import Json.Decode exposing (int, string, succeed)
import Json.Decode.Pipeline exposing (required, decode)

userProfileDecoder : Decoder UserProfile
userProfileDecoder =
    decode UserProfile
        |> required "id" int
        |> required "name" string
        |> required "email" string

{- Use this decoder as before with decodeString to decode JSON strings. -}

This approach simplifies the decoder, making the code cleaner and more maintainable, especially as the data structures become more complex.

Working with CSVWorking with TOML