I am not sure if this is a bug or not, but I have found a program where Hugs (Nov 2003) requires adding a redundant constraint to class methods. Loading this code (it is quite useless, but it is the smallest example I could come up with that gave the same error message as my much larger actual program): class FromStr a where fromStr :: String -> a typeError :: FromStr a => a -> a typeError t = error "type error" instance FromStr [a] where fromStr _ = typeError undefined -- line 8 instance FromStr [(String,a)] where -- line 10 fromStr _ = typeError undefined -- line 11 with "hugs -98 +o test.hs" gives me: ERROR "test.hs":8 - Cannot justify constraints in instance member binding *** Expression : fromStr *** Type : FromStr [a] => String -> [a] *** Given context : FromStr [a] *** Constraints : FromStr [a] Adding the constraint "FromStr a" to the declaration of fromStr fixes the problem, but that seems like it should be redundant. Removing the second instance (lines 10-11) also fixes the problem, interestingly enough. /Bjorn Bringert