InputData.hs

data Controller = Slider | Toggle
data Ref = Value | Ref

type InputData :: Ref -> Type
type InputData ref =
  Struct
    '[ "mousePos"  ':-> V 2 Float
     , "scancodes" ':-> Array 512 Word32
--   , screensize ...
--   , time ...
     , "imguiData" ':-> ImguiData ref
     ]

type family Apply ref a where
  Apply 'Value a = a
  Apply 'Ref   a = ( String, Controller, IORef a )

type ImguiData :: Ref -> Type
type ImguiData ref = 
  Struct
    '[ "slider1" ':-> Apply ref Float
     , "slider2" ':-> Apply ref Float
     ]

type ReadRefs :: [ Symbol :-> Type ] -> [ Symbol :-> Type ] -> Constraint
class ReadRefs as bs | bs -> as, as -> bs where
  readRefs :: Struct as -> IO ( Struct bs )

instance ReadRefs '[] '[] where
  readRefs _ = pure End

instance ( ReadRefs as bs, k1 ~ k2, a ~ ( String, Controller, IORef b ) ) =>
  ReadRefs ( (k1 ':-> a) ': as ) ( (k2 ':-> b) ': bs ) where
  readRefs ( ( _, _, ref ) :& refs ) = do
    val <- readIORef ref
    vals <- readRefs @as @bs refs
    pure ( val :& vals )

class CreateRefs as where
  createRefs :: IO ( Struct as )

instance CreateRefs '[] where
  createRefs = pure End

instance CreateRefs as => CreateRefs ( (k ':-> a) ': as ) where
  createRefs = do
    ref <- newIORef @a
    refs <- createRefs
    

class CreateControllers as where
  createControllers :: IO ( Struct as ) -> IO ()

instance CreateControllers '[] where
  createControllers _ = pure ()

instance ( a ~ ( String, Controller, IORef Float ), CreateControllers as ) => CreateControllers ( ( k ':-> a ) ': as ) where
  createControllers ( (controllerName, controllerType, ref) :& as ) = do
    createController controllerName controllerType ref
    createControllers as

createController :: String -> Controller -> IORef Float -> IO ()
createController controllerName controllerType ref =
  case controllerType of
    Slider -> do
     -- include Dear ImGui code for creating a slider
    Toggle -> do
     -- toggle...

readImguiData :: ImguiData Ref -> IO ( ImguiData Val )
readImguiData = readRefs

zeroInputData :: InputData Val
zeroInputData = V2 0 0 :& Prelude.pure 0 :& imguiStruct :& End
  where
    imguiStruct = ...

{-# NOINLINE zeroInputRefs #-}
zeroInputRefs :: InputData Ref
zeroInputRefs = unsafePerformIO do
  slider1Ref <- newIORef @Float 0
  slider2Ref <- newIORef @Float 0
  let
    imguiStruct :: ImguiData Ref
    imguiStruct
      =  ( "Slider 1", Slider, slider1Ref )
      :& ( "Slider 2", Slider, slider2Ref )
      :& End
  pure $ V2 0 0 :& Prelude.pure 0 :& imguiStruct :& End