Hello Henry, Changes to GHC regarding the treatment of higher-rank types required a few changes to that test too. You have to eta-expand the application of mkTT and give it a type signature. Therefore, main becomes
print $ gzip (\x y -> mkTT maxS x y) genCom1 genCom2 > and you have to add the type signature mkTT :: (Typeable a, Typeable b, Typeable c) => (a -> a -> a) -> b -> c -> > Maybe c . > Then this example should work. As to why exactly these changes are necessary, you might want to check the GHC manual section on arbitrary-rank polymorphism ( http://haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html#universal-quantification ). Cheers, Pedro On Sat, Feb 14, 2009 at 21:55, Henry Laxen <nadine.and.he...@pobox.com>wrote: > Dear Group, > > When trying to run the example at: > http://www.cs.vu.nl/boilerplate/testsuite/gzip/Main.hs > ghc 6.10.1 says > > A pattern type signature cannot bind scoped type variables `a' > unless the pattern has a rigid type context > In the pattern: f :: a -> a -> a > In the definition of `mkTT': > mkTT (f :: a -> a -> a) x y > = case (cast x, cast y) of { > (Just (x' :: a), Just (y' :: a)) -> cast (f x' y') > _ -> Nothing } > In the definition of `main': > main = print $ gzip (mkTT maxS) genCom1 genCom2 > where > genCom1 = everywhere (mkT (double "Joost")) genCom > genCom2 = everywhere (mkT (double "Marlow")) genCom > double x (E (p@(P y _)) (S s)) | x == y = E p (S (2 * s)) > double _ e = e > maxS (S x) (S y) = S (max x y) > .... > Failed, modules loaded: CompanyDatatypes. > > --------------------------------------------------------------------- > I must admit I don't really know what to make of this. Any insights would > be appreciated. > Thanks. > Henry Laxen > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >
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