[Haskell-cafe] Confused by SYB example with zipping

[José Pedro Magalhães]

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.henry at 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
>
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