GHC 6.12.1 and impredicative polymorphism
ben_moseley at mac.com
Fri Oct 30 14:44:06 EDT 2009
Is there any difference between "-XImpredicativePolymorphism" and "-
Hyena uses the latter, and we're using Hyena somewhat "in anger".
On 30 Oct 2009, at 09:51, Simon Peyton-Jones wrote:
> 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 , but is also
> 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.
>  http://research.microsoft.com/en-us/um/people/simonpj/papers/boxy/
>  http://research.microsoft.com/en-us/um/people/crusso/qml/
>  http://research.microsoft.com/en-us/um/people/daan/pubs.html
>  http://gallium.inria.fr/~remy/mlf/
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