cannot build superclass instance

[pablocardenal@laplatavive.com]

Hello, where is the error?

Tested on Hugs 98 February 2001 in Windows 98 Operating System with options -98 +som
===========================================================

>class Result r where
> nullR :: r
> toList :: Result r' => r -> [r']

instance Result [a] where
 nullR = []

-----------------------------------------------

>class Result r => SubResult r  where
> sub :: r

>instance SubResult [a] where   -- this is the line 22
> sub = []

-----------------------------------------------

if add this line 31, then error in line 22 
if remove this line 31, then not error

>instance Result String       -- this is the line 31


Reading file "..\core\result.lhs":
Type checking
ERROR ..\core\result.lhs:22 - Cannot build superclass instance
*** Instance            : SubResult [a]
*** Context supplied    : ()
*** Required superclass : Result [a]    

Obviously if replace line 22 with

instance Result [a] => SubResult [a] where   -- this is
                                          -- the new line 22

no error occurs, but why if remove the line 31 the error
not occurs ?

In previous version of Hugs (March 1999)  this error not
occurs

Thanks.

Pablo de la Hoya Cardenal from Argentina