Is there any difference between "-XImpredicativePolymorphism" and "-
XImpredicativeTypes"?
Hyena uses the latter, and we're using Hyena somewhat "in anger".
--Ben
On 30 Oct 2009, at 09:51, Simon Peyton-Jones wrote:
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/
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