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.