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.ht...
).
Cheers,
Pedro
On Sat, Feb 14, 2009 at 21:55, Henry Laxen
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
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