From af28015f9139151ef33098481e774be92bfcbe8f Mon Sep 17 00:00:00 2001 From: "simonpj@microsoft.com" Date: Fri, 30 Oct 2009 09:59:29 +0000 Subject: [PATCH] Deprecate the ImpredicativeTypes feature 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/ --- compiler/main/DynFlags.hs | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/compiler/main/DynFlags.hs b/compiler/main/DynFlags.hs index d445055..e5a549a 100644 --- a/compiler/main/DynFlags.hs +++ b/compiler/main/DynFlags.hs @@ -1811,7 +1811,8 @@ xFlags = [ ( "LiberalTypeSynonyms", Opt_LiberalTypeSynonyms, const Supported ), ( "Rank2Types", Opt_Rank2Types, const Supported ), ( "RankNTypes", Opt_RankNTypes, const Supported ), - ( "ImpredicativeTypes", Opt_ImpredicativeTypes, const Supported ), + ( "ImpredicativeTypes", Opt_ImpredicativeTypes, + const $ Deprecated "Impredicative polymorphism will be simplified or removed in GHC 6.14" ), ( "TypeOperators", Opt_TypeOperators, const Supported ), ( "RecursiveDo", Opt_RecursiveDo, deprecatedForLanguage "DoRec"), -- 1.7.10.4