Am Freitag, 30. Oktober 2009 10:51:37 schrieb Simon Peyton-Jones: > Friends > > One more update about GHC 6.12, concerning impredicative polymorphism. > > GHC has had an experimental implementation of impredicative polymorphism > for a year or two now (flag -XImpredicativePolymorphism). But > > a) The implementation is ridiculously complicated, and the complexity > is pervasive (in the type checker) rather than localized. > I'm very unhappy about this, especially as we add more stuff to > the type checker for type families. > > b) The specification (type system) is well-defined [1], but is also > pretty complicated, and it's just too hard to predict which programs will > typecheck and which will not. > > So it's time for a re-think. I propose to deprecate it in 6.12, and remove > it altogether in 6.14. We may by then have something else to put in its > place. (There is no lack of candidates [2,3,4]!) > > Fortunately, I don't think a lot of people use the feature in anger. > Please yell if you *are* using impredicative polymorphism for something > serious. But if you are, we need to think of a workaround. The current > situation seems unsustainable. > > Simon > > [1] http://research.microsoft.com/en-us/um/people/simonpj/papers/boxy/ > [2] http://research.microsoft.com/en-us/um/people/crusso/qml/ > [3] http://research.microsoft.com/en-us/um/people/daan/pubs.html > [4] http://gallium.inria.fr/~remy/mlf/
Hello Simon and others, unfortunately, I missed this e-mail. Yes, removal of impredicative polymorphism hurts me, since impredicativity plays a crucial role in the Grapefruit FRP library at the moment. This is described in section 5 of my paper “Signals, Not Generators!” [5]. It’s probably possible to use a workaround involving a newtype wrapper, in case polymorphic fields in newtypes are still supported. However, this makes things more awkward for library users. Best wishes, Wolfgang [5] <http://www.informatik.tu-cottbus.de/~jeltsch/research/tfp-2009-paper.pdf> _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
