How expectJson function works in Elm

ExpectJson.elm

{-
I got frustrated about how Http.get works, so I expanded all the function in Elm core to get to the bottom of line
-}
expectJson : (Result Error a -> msg) -> Decode.Decoder a -> Expect msg
expectJson toMsg decoder =
  let
    decodingResult string =
      case (Decode.decodeString decoder string) of
        Ok v ->
          Ok v

        Err e ->
          Err (Decode.errorToString e)
          
    toResult: r -> Result Http.Error a
    toResult response =
      case response of
        BadUrl_ url           -> Err (BadUrl url)
        Timeout_              -> Err Timeout
        NetworkError_         -> Err NetworkError
        BadStatus_ metadata _ -> Err (BadStatus metadata.statusCode)
        GoodStatus_ _ body    ->
                case (decodingResult body) of
                  Ok v -> Ok v
                  Err e -> Err (BadBody e)
  in
  Elm.Kernel.Http.expect "" identity (toResult >> toMsg)
  
  type Msg = 
    GotPerson Result Http.Error Person
  
  Http.get 
    { url = "http://swapi.com/person/1"
    , expect = Http.expectJson GotPerson decodePerson
    }