After some fooling around, I came up with something I think makes sense. Let me know if this is the right/wrong thing. It seems to work for the examples I've tried so far.
instance (Floating f, MetricSpace e f ,MetricSpace e' f, HZip l l (HCons (e', e') l') ,HFoldr ApplyDistSum Float l' f) => MetricSpace (HCons e l) f where c `dist` c' = sqrt $ hFoldr ApplyDistSum (0::Float) (hZip c c')
It seems strange that you need the types e and e' (perhaps this is a quirk or a bug of GHC 6.8). With GHC 6.6, I have derived the following instance (Floating f, MetricSpace e f, HFoldr ApplyDistSum Float l1 f, HZip (HCons e l) (HCons e l) (HCons (e,e) l1)) => MetricSpace (HCons e l) f where c `dist` c' = sqrt $ hFoldr ApplyDistSum (0::Float) (hZip c c') which matches my intuitive understanding, and also sufficient to run the given examples. When I wrote `I derived with GHC' I meant it literally. First I wrote the instance without any constraints: instance () => MetricSpace (HCons e l) f where c `dist` c' = sqrt $ hFoldr ApplyDistSum (0::Float) (hZip c c') GHC of course complained about many missing constraints. I started adding the constraints from the list of complaints, until GHC was satisfied. This is basically a cut-and-paste job from the Emacs buffer with GHC error messages to the buffer with the code.