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