[Haskell-cafe] Confused by SYB example with zipping

[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.

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I must admit I don't really know what to make of this.  Any insights would
be appreciated.
Thanks.
Henry Laxen