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Simon Peyton Jones authored
GHC has had an experimental implementation of impredicative polymorphism for a year or two now (flag -XImpredicativeTypes). 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]!) [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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