Line 24 is where I am confused

OCanvas.hs

mkCanvas' :: CanvasInfo -> IO (OCanvasRef)
mkCanvas' cnvInfo = do
  cnvObj <- toJSRef cnvInfo
  cnvRef <- js_create cnvObj
  js_setWidth cnvRef (toJSInt $ width cnvInfo)
  js_setHeight cnvRef (toJSInt $ height cnvInfo)
  return cnvRef


mkCanvas :: (MonadWidget t m) => Event t CanvasInfo -> m (Event t OCanvasRef)
mkCanvas cnvInfoE = do
  performEvent $ fmap (liftIO . mkCanvas') cnvInfoE


canvasDisplay' :: OCanvasRef -> ImageInfo -> IO (OImageRef)
canvasDisplay' cnvRef imgInfo = do
  imgInfoRef <- toJSRef imgInfo
  imgRef <- js_display cnvRef imgInfoRef
  return imgRef


canvasDisplay :: (MonadWidget t m) => Event t OCanvasRef -> Dynamic t ImageInfo -> m (Event t OImageRef)
canvasDisplay cnvRefE imgInfoD = do
  cnvRefD <- foldDynMaybe notNull jsNull cnvRefE
  let cnvImgE = attachDyn cnvRefD (updated imgInfoD)
  performEvent $ fmap (liftIO . canvasDisplay') cnvImgE