Hello All, Defining a partially ordered set as: data Porder = HI | LW | NT deriving (Eq) class (Eq a) => Poset a where pcompare :: a -> a -> Porder and defining an instance for type synonym RuRe: type RuRe = ([Int],[Int]) instance Poset RuRe where pcompare (x1,y1) (x2,y2) | isSub x2 x1 = HI | isSub x1 x2 = LW | otherwise = NT causes no problems in Hugs (-98 flag) but compiling with GHC produces: [hvt@localhost Sandbox]$ ghc --make Main.hs -O [5 of 6] Compiling Abduce ( Abduce.hs, Abduce.o ) Abduce.hs:110:0: Illegal instance declaration for `Poset RuRe' (The instance type must be of form (T a b c) where T is not a synonym, and a,b,c are distinct type variables) In the instance declaration for `Poset RuRe' I've tried several changes, like not using the type synonym, but can't get it right. (I'm using the ghc 6.6.1 Fedora Core 6 package). Many thanks in advance! Hans van Thiel