Decoding polymorphic JSON objects into elm with andThen - json

Question

My JSON looks similar to this:
{ "items" :
[ { "type" : 0, "order": 10, "content": { "a" : 10, "b" : "description", ... } }
, { "type" : 1, "order": 11, "content": { "a" : 11, "b" : "same key, but different use", ... } }
, { "type" : 2, "order": 12, "content": { "c": "totally different fields", ... } }
...
]
}
and I want to use the type value to decide what union type to create while decoding. So, I defined alias types and decoders for all the above in elm :
import Json.Decode exposing (..)
import Json.Decode.Pipeline exposing (..)
type alias Type0Content = { a : Int, b : String }
type alias Type1Content = { a : Int, b2 : String }
type alias Type2Content = { c : String }
type Content = Type0 Type0Content | Type1 Type1Content | Type2 Type2Content
type alias Item = { order : Int, type : Int, content: Content }
decode0 = succeed Type0Content
|> requiredAt ["content", "a"] int
|> requiredAt ["content", "b"] string
decode1 = succeed Type1Content
|> requiredAt ["content", "a"] int
|> requiredAt ["content", "b"] string
decode2 = succeed Type2Content
|> requiredAt ["content", "c"] string
decodeContentByType hint =
case hint of
0 -> Type0 decode0
1 -> Type1 decode1
2 -> Type2 decode2
_ -> fail "unknown type"
decodeItem = succeed Item
|> required "order" int
|> required "type" int `andThen` decodeContentByType
Can't get the last two functions to interact as needed.
I've read through page 33 of json-survival-kit by Brian Thicks, but that didn't bring me on track either.
Any advice and lecture appreciated!

Answer 1

It looks like the book was written targeting Elm 0.17 or below. In Elm 0.18, the backtick syntax was removed. You will also need to use a different field name for type since it is a reserved word, so I'll rename it type_.
Some annotations might help narrow down bugs. Let's annotate decodeContentByType, because right now, the branches aren't returning the same type. The three successful values should be mapping the decoder onto the expected Content constructor:
decodeContentByType : Int -> Decoder Content
decodeContentByType hint =
case hint of
0 -> map Type0 decode0
1 -> map Type1 decode1
2 -> map Type2 decode2
_ -> fail "unknown type"
Now, to address the decodeItem function. We need three fields to satisfy the Item constructor. The second field is the type, which can be obtained via required "type" int, but the third field relies on the "type" value to deduce the correct constructor. We can use andThen (with pipeline syntax as of Elm 0.18) after fetching the Decoder Int value using Elm's field decoder:
decodeItem : Decoder Item
decodeItem = succeed Item
|> required "order" int
|> required "type" int
|> custom (field "type" int |> andThen decodeContentByType)