On Jan 28, 2008 12:45 AM,
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.
This also works in GHC 6.8. Thanks!
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.
Wow. I will try this next time I post. Thanks very much. -- Denis