Hi, While playing with type families in GHC 6.10.1, I guess I bumped into the no-overlap restriction. As I couldn't find any examples on that, I include the following (non-compiling) code so as to check with you if that's really the case: ------------------------------------------- {-# OPTIONS -fglasgow-exts #-} {-# LANGUAGE OverlappingInstances #-} {-# LANGUAGE UndecidableInstances #-} module Main where class Expr t where type ExprRslt t :: * eval :: t -> ExprRslt t instance Expr t where type ExprRslt t = t -- overlap? eval = id data Vector a = Vector {width :: !Int, dat :: [a]} data Subscript a = Subscript {vec :: (Vector a), ind :: !Int} instance Expr (Subscript a) where type ExprRslt (Subscript a) = a eval sub = (dat.vec $ sub) !! ind sub ------------------------------------------------ So this means that classes with associated types cannot have default instances at all? If so, could you possibly refer me to any material explaining why? Thanks in advance. Cheers, Jorge.