Decode json object with warning for unhandled keys

[rofrol]

This is my attempt to decode keys but get warning when some key is not handled. Full example https://ellie-app.com/mxkGY5DVFa1/0.

Running this code will give this in browser console: [Warning]: I ran into afaildecoder: Unhandled field somefield: { url = "", amount = 0 }

Thanks @jessta for pointing to keyValuePairs. The code is base on this http://folkertdev.nl/blog/elm-messy-json-value/.

How can I make this more generic (less MyRecord) so I can reuse more code?
What can be improved, done differently?

module Main exposing (main)

import Html exposing (Html, text)
import Json.Decode exposing (..)


(|:) : Decoder (a -> b) -> Decoder a -> Decoder b
(|:) =
    map2 (<|)


serverResponseMy : String
serverResponseMy =
    """
    {
        "url": "https://elm-lang.org",
        "amount": 1.2,
        "somefield": 1
    }
"""


partialDecodeField : Decoder (List ( String, Value ))
partialDecodeField =
    keyValuePairs value


fromResultWithWarning : (String -> b -> Decoder b) -> ( String, Value ) -> b -> b
fromResultWithWarning decoder ( name, value ) record =
    case decodeValue (decoder name record) value of
        Ok newRecord ->
            newRecord

        Err error ->
            Debug.log ("[Warning]: " ++ error) record


feedNameL : (String -> MyRecord -> Decoder MyRecord) -> List ( String, Value ) -> MyRecord
feedNameL decoder elements =
    List.foldl (fromResultWithWarning decoder) defaultMyRecord elements


decodeList : Decoder MyRecord
decodeList =
    map (feedNameL decodeField) partialDecodeField


decodeField : String -> MyRecord -> Decoder MyRecord
decodeField name record =
    case name of
        "url" ->
            succeed (\url -> { record | url = url }) |: string

        "amount" ->
            succeed (\amount -> { record | amount = amount }) |: float

        _ ->
            fail <| "Unhandled field " ++ name


type alias MyRecord =
    { url : String
    , amount : Float
    }


defaultMyRecord : MyRecord
defaultMyRecord =
    MyRecord "" 0

main : Html msg
main =
    text <| toString <| decodeString decodeList serverResponseMy

[rupert]

I have a feeling there is a library package for this already, as I remember asking about it before. I’ll see if I can dig back on the old elm-discuss list and find it.

This one, I think:

http://package.elm-lang.org/packages/zwilias/json-decode-exploration/latest

https://groups.google.com/forum/#!searchin/elm-discuss/json$20missing$20fields|sort:date/elm-discuss/-WJRY3r8TDs/R57qsuKCAwAJ

[rofrol]

Thanks, though I know about this library and strict mode. I wanted to do this with standard library as this is what you have to deal with in examples, libraries etc.