type families and overlapping

[Jorge Marques Pelizzoni]

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.
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