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" Triangle (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

main = render $ \(V4 r g b a) -> V4 0 0 b 1