Why is there no Zippable class? There is.
You can use Data.Zippable from http://hackage.haskell.org/package/bff.
It gives you a function
tryZip :: Zippable k => k a -> k b -> Either String (k (a,b))
The Either in the return type is to capture an error message in case the two structures are not of the same shape.
This functionality can also be obtained from the generic programming library EMGM, with the function zip :: FRep3 ZipWith f => f a -> f b -> Maybe (f (a, b)) You can use Template Haskell to generate the necessary FRep3 instances. Once you have those you get many other generic functions for free. See http://hackage.haskell.org/package/emgm -- Johan Jeuring
For example, for
data Tree a = Leaf a | Node (Tree a) (Tree a)
you would have:
instance Zippable Tree where tryZip (Leaf a) (Leaf b) = Right (Leaf (a,b)) tryZip (Node a1 a2) (Node b1 b2) = do z1 <- tryZip a1 b1 z2 <- tryZip a2 b2 return (Node z1 z2) tryZip _ _ = Left "Structure mismatch."
Of course, you can get an "unsafe" zip by composing tryZip with a fromRight.
What's more, the mentioned package contains an automatic Template Haskell deriver for Zippable instances, so you don't have to write the above instance definition yourself.
The implementation is by Joachim Breitner.
Ciao, Janis.
-- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de
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