Re: impredicative polymorphism: seen on HCAR

I wrote:

A side-effect is for those of us who want to have any concern for future compatibility with non-GHC type systems (or the long future of GHC's, even), whether one of the proposals would be likely to be easier for someone else to implement in a different compiler.

actually, just as important is the ability of the GHC implementation to turn on and off (-XImpredicativeTypes?) so that we can use GHC to make sure our programs don't go beyond, e.g., Rank2Types if we don't want them to. -Isaac