time = Uniform "Time" :: Float
clear = FrameBuffer $ ColorImage @1 $ V4 1 0 0 0.5

triangleRasterCtx = TriangleCtx CullNone PolygonFill NoOffset LastVertex
colorFragmentCtx = accumulationContext ((ColorOp NoBlending (V4 True True True True)))

rasterizeWith = Rasterize
triangles = triangleRasterCtx

quadVertexStream = fetch "quad" (Attribute "position" :: ((Vec 4 Float)))

transform s f =  Transform (\v -> VertexOut (f v) 1 () (Smooth v)) s

mapFragments s fs = accumulate colorFragmentCtx PassAll (FragmentShader $ \((a)) -> ((fs a))) s clear

render f = quadVertexStream                 -- id vertices
    `transform`    id
     &             rasterizeWith triangles  -- rasterize
    `mapFragments` f
     &             ScreenOut                --  draw into screen

fun x = if x < 0 then 0 else x +! fun (x -! 0.3)

main' = render $ \c -> if fun time < 0.5 then c else blue