-- required pre-existing state: A window context, a bound vertex array object, and an active shader.
-- Does not clear the back buffer or swap it to the front, only adds extra data to it.
drawShape :: (VertexAttribComponent v, Storable v) => [VertexAttribData v] -> PrimitiveMode -> IO ()
drawShape vertexAttribs mode = drawShapeR vertexAttribs mode 0 []
  where drawShapeR [] _ _ _ = putStrLn "drawShape encountered empty vertex list, doing nothing"
        drawShapeR (headAttribs:tailAttribs) mode n buffers = do
          vertexBuffer <- genObjectName
          bindBuffer ArrayBuffer $= (Just vertexBuffer)
          let vertexArrayLength = numVertices headAttribs
          (size, bufPtr) <- getVertexAttribArray headAttribs
          bufferData ArrayBuffer $= (fromIntegral (vertexArrayLength * size), bufPtr, StreamDraw)

          vertexAttribPointer (AttribLocation n) $=
            (ToFloat, VertexArrayDescriptor
                      (numAttributeComponents headAttribs)
                      Float
                      0
                      (plusPtr nullPtr . fromIntegral $ 0))
          vertexAttribArray (AttribLocation n) $= Enabled
          -- if all vertex attributes have been processed, draw the object and clean up the buffers
          case tailAttribs of [] -> drawArrays mode 0 (fromIntegral vertexArrayLength) >>
                                    forM_ buffers deleteObjectName
                              _  -> drawShapeR tailAttribs mode (n + 1) (vertexBuffer:buffers)

data VertexAttribData a = PosData2D NumComponents [Vertex2 a]
                        | ColorData NumComponents [Color4 a]
                        | TexUVData NumComponents [TexCoord2 a]
                        deriving( Eq, Show, Read )

numAttributeComponents :: VertexAttribData a -> NumComponents
numAttributeComponents (PosData2D n _) = n
numAttributeComponents (ColorData n _) = n
numAttributeComponents (TexUVData n _) = n

numVertices            :: VertexAttribData a -> Int
numVertices (PosData2D _ vertices) = length vertices
numVertices (ColorData _ vertices) = length vertices
numVertices (TexUVData _ vertices) = length vertices


getVertexAttribArray :: (Storable a) => VertexAttribData a -> IO (Int, Ptr a)
getVertexAttribArray (PosData2D _ vertices) = newArray vertices
  >>= (\ptr -> (peekElemOff ptr 0 >>=
                 (\fstElem -> return (sizeOf fstElem, castPtr ptr))))
getVertexAttribArray (ColorData _ vertices) = newArray vertices
  >>= (\ptr -> (peekElemOff ptr 0 >>=
                 (\fstElem -> return (sizeOf fstElem, castPtr ptr))))
getVertexAttribArray (TexUVData _ vertices) = newArray vertices
  >>= (\ptr -> (peekElemOff ptr 0 >>=
                 (\fstElem -> return (sizeOf fstElem, castPtr ptr))))