Diffuse shading with shadows

diffuseM.hs

diffuseM :: Colour -> Material () (BSDF Colour)
diffuseM col = proc () -> do
    out <- diffuse -< col                                                     -- Get diffuse shading
    shad <- traceM <<< getInidentRay -< ()                                    -- Test path to light
    returnA -< maybe holdout (\(_,_,_,e) -> if e then out else holdout) shad  -- Set BSDF to black if in shadow