Json

module Json exposing (..)

{-|
This module is intended to explore an alternative API to Json.
@docs decodeField
-}

import Json.Encode
import Json.Decode as Json


{-|
    Decode a field using a given setter to get it out of an object. This function is designed to be chained!

    >>> import Json.Decode
    >>> import Json.Encode
    >>> decodeField
    ... "name"
    ... Json.Decode.string
    ... (\model name -> { model | name = name })
    ... (Json.Encode.object [ ("name", Json.Encode.string "noah") ])
    ... (Ok { name = "" })
    Ok { name = "noah" }

    >>> decodeField
    ... "name"
    ... Json.Decode.string
    ... (\model name -> { model | name = name })
    ... (Json.Encode.object [ ("age", Json.Encode.int 18) ])
    ... (Ok { name = "" })
    Err "Expecting an object with a field named `name` but instead got: {\"age\":18}"

    >>> decodeField
    ... "name"
    ... Json.Decode.string
    ... (\model name -> { model | name = name })
    ... (Json.Encode.object [ ("age", Json.Encode.int 18) ])
    ... (Err "Something failed!")
    Err "Something failed!\nAnd Expecting an object with a field named `name` but instead got: {\"age\":18}"
-}
decodeField :
    String
    -> Json.Decoder a
    -> (model -> a -> model)
    -> Json.Value
    -> Result String model
    -> Result String model
decodeField fieldName decoder setter value model =
    let
        decoded =
            Json.decodeValue (Json.field fieldName decoder) value
    in
        case model of
            Err s ->
                case decoded of
                    Err newMessage ->
                        Err (s ++ "\nAnd " ++ newMessage)

                    Ok _ ->
                        model

            Ok v ->
                Result.map (setter v) decoded