Custom JSON Decoders

Introduction#

How to use Json.Decode to create custom decoders, for example decoding into union types and user defined data types

Decoding into union type

import Json.Decode as JD
import Json.Decode.Pipeline as JP

type PostType = Image | Video
 
type alias Post = { 
    id: Int
    , postType: PostType
}
-- assuming server will send int value of 0 for Image or 1 for Video
decodePostType: JD.Decoder PostType
decodePostType = 
    JD.int |> JD.andThen (\postTypeInt -> 
        case postTypeInt of 
            0 ->
                JD.succeed Image


            1 ->
                JD.succed Video

            _ ->
                JD.fail "invalid posttype"

    )

decodePostMap : JD.Decoder Post
decodePostMap = 
    JD.map2 Post
        (JD.field "id" JD.int)
        (JD.field "postType" decodePostType)

decodePostPipline : JD.Decoder Post
decodePostPipline = 
    JP.decode Post
        |> JP.required "id" JD.int
        |> JP.required "postType" decodePostType