I have a problem with defining an instance.
module CollTree where
import Data.List(intersperse)
Say, I want to define a tree structure based on lists. I omit the data attached to the nodes here for simplicity.
data ListTree = ListNode [ListTree] deriving Show
This is simple enough for an automatically derived Show instance. Now I want to generalise that structure from lists to arbitrary types of collections.
data CollTree coll = CollNode (coll (CollTree coll))
This got too complicated for automatic derivation. So I have to define a Show instance for CollTree manually. But how to formulate the instance context? How to avoid undecidable instances? instance ??? => Show (CollTree coll) where show (CollNode xs) = "CollNode " ++ show xs Currently I see only one solution using a new class providing a special 'show' routine, which handles the structure of the collection, but must be assisted by a custom 'show' function for the collection members.
class CollShow coll where collShow :: (a -> String) -> (coll a -> String)
instance CollShow coll => Show (CollTree coll) where show (CollNode xs) = "CollNode " ++ collShow show xs
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?