elm decoder very helpful to debug decoders used in event handlers (thanks @JoelQ on the elm slack)

gistfile1.txt

module Main exposing (main)

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


loggingDecoder : Decoder a -> Decoder a
loggingDecoder realDecoder =
    Decode.value
        |> Decode.andThen
            (\value ->
                case Decode.decodeValue realDecoder value of
                    Ok decoded ->
                        Decode.succeed decoded

                    Err error ->
                        Decode.fail <| Debug.log "decode error" error
            )


nameDecoder : Decoder String
nameDecoder =
    Decode.field "name" Decode.string


json : String
json =
    """
  { "age": 42 }
  """


main : Html msg
main =
    case Decode.decodeString (loggingDecoder nameDecoder) json of
        Ok name ->
            text name

        Err _ ->
            text "check the logs for the error"

loggingDecoder realDecoder =
    D.value
        |> D.andThen
            (\value ->
                case D.decodeValue realDecoder value of
                    Ok decoded ->
                        D.succeed decoded

                    Err error ->
                        D.fail <| D.errorToString <| Debug.log "decode error" error
            )

D.fail <| D.errorToString <| Debug.log "decode error" error