Niall Douglas
Andrew's Concepts Lite let you much more richly specify partial template specialisations - in fact, PTS looks very clunky compared to what Concepts Lite allows. That certainly helps with the brevity and power of implementation, but it also helps with the sanity of usage. Whilst use of the existing MPL is fairly problem free, that is definitely not the case for stuff which is based on the MPL where you'll find the metaprogramming becomes very brittle a la Boost.Spirit. I suspect, without evidence, that correctly and permanently eliminating that brittleness will require Concepts Lite in the MPL layer, not in the layer above.
Thanks for clarifying. I'll take a deeper look at Concepts Lite and see how this might relate to the MPL (and also to my work so far). Louis