[Haskell-cafe] Tree with collections

[Henning Thielemann]

On Sat, 11 Mar 2006, Roberto Zunino wrote:

> 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

Yes, this solves the problem. Thanks a lot!

> 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).

I tried similar things with GHC 6.2 and Hugs and they looped.