Hello Simon, i write to you, Simon (not to Igloo), because these changes require a knowledge of new GHC language features that you have implemented second paragraph of release notes (http://www.haskell.org/ghc/dist/current/docs/users_guide/release-6-6.html) is still erroneous. it mention two _different_ things - addition of impredicative polymorphism and changes in scoped type variables. Your mail mentioned here is about variables while section 7.4.9 is about polymorphism. There is also section 7.4.10 what say about variables and in my POV is much more readable than your letter. so i propose to write smth like the: - GHC now handles impredicative polymorphism, see Section 7.4.9, ⌠Impredicative polymorphism■ for the documentation - There are changes to the way scoped type variables work, see Section 7.4.10 for details. Shortly saying, the following is no more works: f (x::a) = sizeOf (undefined::a) g :: a = genericLength [] `asTypeOf` (undefined::a) while Section 7.4.10 should mention "Simon's letter" as containing more technical details. i also propose to add here explanations of how old code can be converted to new syntax. it may be obvious for you but not for us, common haskellers :D f (x::a) = sizeOf (undefined::a) should be now written as f :: a -> Int f (x::a) = sizeOf (undefined::a) to reach compatibility with old GHC versions and Hugs. well, i'm not language lawyer and not sure what is the best way, but at least using 'forall' to bring 'a' into scope is not H98-compatible It's very important to mention that this change means _dropping_ capabilities, not adding new ones, because it is completely unobvious from current docs. i don't realized it until i've compiled my program and spend several hours trying to understand 7.4.10, your letter and then 7.4.10 again :) i hope that proposed changes will greatly simplify this "learning curve" :) -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com