Henning Thielemann wrote:
Now I have to write 'show' related code for each collection type. This
way I probably duplicate a lot of code that is already written for the
Show instances of the collections. To be honest, I use a more special
tree structure with even more special "collections" so this may be not
really a problem.
Here an instance for a custom list type:
newtype NewList a = NewList [a]
instance CollShow NewList where
collShow shw (NewList xs) =
"(NewList [" ++
concat (intersperse ", " (map shw xs)) ++ "])"
Is there a more straightforward way, preferrably Haskell98?
This avoids duplicating code between Show/ShowColl .
> instance ShowColl coll => Show (CollTree coll) where
> show (CollNode n) = "CollNode " ++ showColl n
>
> class ShowColl coll where
> showColl :: coll (CollTree coll) -> String
>
> instance ShowColl [] where
> showColl = show
Also, with GHC extensions and undecidable instances, the following
incantation seems to work:
> {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-}
> module CollTree where
> data CollTree coll = CollNode (coll (CollTree coll))
>
> instance Show (coll (CollTree coll)) => Show (CollTree coll) where
> show (CollNode n) = "CollNode " ++ show n
*CollTree> show (CollNode [CollNode [],CollNode []])
"CollNode [CollNode [],CollNode []]"
However, I can not figure why the typechecker does not loop here (GHC
6.4.1).
Regards,
Roberto Zunino.
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