Apple's Core Foundation data structures do this sort of "usage-based implementation selection," though in a much simpler context: (see e.g. http://ridiculousfis

Apple's Core Foundation data structures do this sort of "usage-based implementation selection," though in a much simpler context: (see e.g. http://ridiculousfish.com/blog/archives/2005/12/23/array/ http://ridiculousfish.com/blog/archives/2005/12/23/array/). I'm not sure why this technique isn't more widespread.