data AB = A | B

render choice = let
  emptyFB           = FrameBuffer (colorImage1 (V4 0.0 0.0 0.4 1.0))
  rasterCtx         = TriangleCtx CullNone PolygonFill NoOffset LastVertex
  fragmentCtx       = (ColorOp NoBlending (V4 True True True True))
  vertexShader v    = (v, v)
  vertexStream      = fetch "stream4" Triangle (Attribute "position4" :: Vec 4 Float)
  primitiveStream   = mapPrimitives vertexShader vertexStream
  fragmentStream    = rasterizePrimitives rasterCtx Smooth primitiveStream
  fragmentShader    = \v -> case choice of
                            A -> v
                            B -> V4 1.0 0.0 0.0 1.0
  frame             = accumulate fragmentCtx fragmentShader fragmentStream emptyFB
 in ScreenOut frame

data Out = Out (AB -> Output)

out = Out render

main = let
  Out o = out
  in o A