[Haskell-cafe] Why is there no Zippable class? Would this work?

[Johan Jeuring]

> 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 at tcs.inf.tu-dresden.de
>
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe