This is pretty fascinating, although superficially a lot of this seems more complex than just plain opengl. For example:

  clear = FrameBuffer $ (DepthImage @1 1000, ColorImage @1 navy)   -- ...
  mapFragments s fs = Accumulate colorFragmentCtx PassAll (\a -> FragmentOutRastDepth $ fs a) s clear

Compared with:

  glClearColor(...)
  glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT)

State management is a nightmare in OpenGL, but that second chunk of code is a lot easier to wrap my head around even if it's objectively worse in a lot of ways. Still though, I'm waiting to see the language tutorial before I make up my mind. Looks interesting!