[ats-lang-users] Can you pass !(&viewtypevar) as an argument that can be mutated?

[Hans V. <keg...@gm...>]

Hello Dr. Xi & friends,

I am a very stupid man when it comes to types, and I'm working on my
first ATS project using SDL and Emscripten. It worked right away,
which felt amazing! However, I'm trying to understand how I can pass
around a var like I do !type to avoid consuming the linear proof and
still be able to mutate the argument. (Perhaps assuming this makes
sense is my mistake.)

I found a few posts on this list about creating "takeout/putback"
prvals, as well as this excellent post
https://bluishcoder.co.nz/2011/04/25/sharing-linear-resources-in-ats.html
which came very close, but unfortunately I was unable to figure out
how to apply it to my problem of passing around a reference to a var
of a viewtype whose proofs I want to avoid consuming while still being
able to mutate the argument.

The below compiles if I don't attempt to set any field of ui, but If I
uncomment either the ui.dimensions or ui.running :=, I get an error
that seems to indicate that the l1 proof for ui was consumed.

10062(line=333, offs=5) -- 10064(line=333, offs=7): error(3): unsolved
constraint: C3NSTRprop(C3TKmain(); S2Eeqeq(S2Evar(l1$9081(14901));
S2Evar(l1$9028(14847))))
src/main.dats: 10062(line=333, offs=5) -- 10064(line=333, offs=7):
error(3): unsolved constraint for var preservation
typechecking has failed: there are some unsolved constraints: please
inspect the above reported error message(s) for information.

This makes sense to me since I'm passing &ui, not !ui. If I change the
argument of oneLoop to !ui (whether ui is an @ or '), I get this
error:

9865(line=324, offs=34): error(3): a non-proof component is replaced
of the type: [S2Ecst(bool)].

I assume this is because the argument is no longer mutable.

Should I not be trying to use a viewtype var this way?

Thank you in advance,
Hans

PS: Bonus question: Is there a way in ATS to do the matching of
SDL_Event below as a switch statement (i.e. pattern-matching on
$extval constants)?

---

typedef dimensions =
 @{ width = int
  , height = int
  , refreshRate = int
  }

viewtypedef ui (l1 : addr, l2 : addr, l3 : addr) =
 @{ window = SDL_Window l1
  , renderer = SDL_Renderer l2
  , font = TTF_Font l3
  , dimensions = dimensions
  , running = bool
  }

viewtypedef ui = [l1, l2, l3 : agz] ui(l1, l2, l3)

// (SDL_Window/Renderer/TTF_Font are addr absviewtypes + agz
viewtypedef as you'd expect)

typedef callback = (&ui) -<fun1> void

extern fun emscripten_set_main_loop_arg (func : callback, arg : &ui,
fps : int, simulate_infinite_loop : bool) : void = "mac#"

fun oneLoop (ui : &ui) : void =
  let
    var e : SDL_Event
    val () = e.type := $Unsafe.cast 0
    var pollRet : int = SDL_PollEvent e
    val () =
      while (pollRet = 1) (
        let
          val () =
            if e.type = SDL_MOUSEMOTION
              then ()
              else if e.type = SDL_WINDOWEVENT
                then () // ui.dimensions := getWindowDimensions ui.window
                else if e.type = SDL_QUIT
                  then () // ui.running := false
                  else ()
        in
          pollRet := SDL_PollEvent e
        end
      )
    // val () = draw ui
  in
    ()
  end

implement main0 () =
  let
   ...
    var ui : ui (null, null, null)
    val () = ui.running := true
    val () = ui.dimensions := getDesktopDimensions ()
    val () = ui.window := SDL_CreateWindow ("risk", 0, 0,
ui.dimensions.width, ui.dimensions.height,
SDL_WINDOW_SHOWN_RESIZABLE_OPENGL_HIGHDPI)
    val () = ui.renderer := SDL_CreateRenderer (ui.window, ~1,
SDL_RENDERER_ACCELERATED)
    val () = ui.font := TTF_OpenFont ("fonts/UbuntuMono-R.ttf", 64)
    val ((* main loop *)) = emscripten_set_main_loop_arg(oneLoop, ui, 0, true)
    val () = SDL_DestroyRenderer ui.renderer
    val () = SDL_DestroyWindow ui.window
    val () = TTF_CloseFont ui.font
    val () = TTF_Quit ()
    val () = SDL_FreeCursor cursorArrow
    val () = SDL_Quit ()
  in
    ()
  end