| Well, Ord Foo doesn't hold, does it? So Ord (a, b) isn't equivalent to (Ord a, Ord b). Indeed not. But that was not what Tom was suggesting (although he was not very explicit). What he meant was this: If you write the instance instance Theta => Foo (T a b) where ... where Theta is an arbitrary context, then, given a dictionary for (Foo (T a b)), you can get dictionaries for each constraint in Theta. Or in logic Theta <=> Foo (T a b) In your example, Theta is empty. If you'd written instance (Show a, Ix b) => Ord (a,b) then you could get a Show a and Ix b dictionaries from an Ord (a,b) dictionary. (But not an Ord a or Ord b one.) S | | Ganesh | | -----Original Message----- | From: Simon Peyton-Jones [mailto:simonpj@microsoft.com] | Sent: 27 March 2008 09:05 | To: Sittampalam, Ganesh; 'Tom Schrijvers'; Ganesh Sittampalam | Cc: glasgow-haskell-users@haskell.org; Martin Sulzmann | Subject: RE: flexible contexts and context reduction | | Why "unfortunately"? Looks fine to me. | | Simon | | | | | Unfortunately, GHC accepts the following: | | | | {-# LANGUAGE FlexibleInstances #-} | | module Foo2 where | | | | data Foo = Foo | | deriving Eq | | | | instance Ord (Foo, Foo) where | | (Foo, Foo) < (Foo, Foo) = False | | ============================================================================== | Please access the attached hyperlink for an important electronic communications disclaimer: | | http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html | ==============================================================================