Dan,
I've been playing around with type families off and on in 6.8, but, what with the implementation therein being reportedly incomplete, it's hard to know what I'm getting right and wrong, or what should work but doesn't and so on. So, I finally decided to take the plunge and install 6.9 (which, perhaps, isn't yet safe in that regard either, but, such is life :)). But, when I loaded up one of my programs, I got a type error. The subject is inductively defined tuples: [..] However, proj results in a type error:
Occurs check: cannot construct the infinite type: t = Lookup (t ::: ts) fn In the pattern: v ::: vs In the definition of `proj': proj FZ (v ::: vs) = v
Sorry, but looks like a bug in 6.9 to me. Could you add it to the GHC bug tracker? Thanks, Manuel