Update Functions

[dmattia]

I am wondering your thoughts on creating chainable updateX methods for each message. This is something I often do in elm for types I create.

There is a history of such methods on proto objects:

• Java's protobuf Builder objects have chainable setX methods.
• C++/JavaScript/PHP defaults to having setX methods, though they are unchainable.
• Python/Ruby allow direct access with mutation
• Objective-C makes the properties readwrite

Example:

message Model {
  string string_field = 1;
  int64 long_field = 2;
}

Example Proto Output (only new functions shown):

updateStringField : String -> Model -> Model
updateStringField newValue record =
  { record | stringField = newValue }

updateLongField : Int -> Model -> Model
updateLongField newValue record =
  { record | longField = newValue }

Example Usage:

update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
  case msg of
    SomeMsg someStr someLong ->
      ( model
          |> updateStringField someStr
          |> updateLongField someLong
      , Cmd.none )

I would gladly offer to help with this if you're interested in the idea

[tiziano88]

Thanks for offering your advice and willing to help @dmattia ! I am not sure whether introducing additional combinators for this kind of updates is any more readable than just using the plain record update syntax.

In your example, the "plain" version would look like

update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
  case msg of
    SomeMsg someStr someLong ->
      ( { model
        | stringField = someStr
        , longField = someLong
        }
      , Cmd.none )

which seems pretty readable to me?

[dmattia]

It does to me as well. The bigger issue that I am hoping to help with is updating nested records, where helper functions can make code considerably more concise and readable (subjective).

I really like the asXIn syntax alongside the setX methods, as described here: https://medium.com/elm-shorts/updating-nested-records-in-elm-15d162e80480

[tiziano88]

I feel the same about nested records, and I wish elm had something like the lens package in Haskell for that. The closest I could find is https://github.com/bChiquet/elm-accessors , perhaps we could add support for generating those combinators in proto, but AFAICT the elm community (or at least @evancz) seems to discourage that pattern (see the message when typing lens into https://package.elm-lang.org/).

[dmattia]

That's fair. I'm convinced enough that this isn't needed for now then.