Hi David.
Now, if I fiddle in the .tix file by hand to fake a usage of (/=) by changing the 52nd entry from 0 to 1, the Eq instance isn't highlighted any more in the HTML output. More strangely, if I then remove the usage of (==) by changing the 51st entry from 1 to 0, the Eq instance still isn't highlighted. A similar effect happens with the Show instance. It seems to be highlighting based only on the last entry in the .tix file, where there are two or more identically-placed boxes. Is this right? I'd have expected that if I use (==) then the deriving (Eq) clause should be considered 'used'.
I've not looked at the .tix file, but a few tests seem to confirm what I'd suspect. For derived instances, you have to cover *all* methods, otherwise the type class will be shown as not covered. Now, in the case of Eq that's both (==) and (/=), where the derived implementation of (/=) happens to use the derived implementation of (==). So using (==) alone is not sufficient, but using (/=) is. Similarly for Show, where the class defines "showList" that you don't test.
Secondly, I can't work out what the four boxes in position 15:6-15:9 are supposed to be. If I use -ddump-simpl I can see many calls to 'tick' but there's no mention of numbers 47, 49 or 50. Perhaps they've been simplified away? I'm afraid I don't know what else to try dumping to get at the instrumented code before the simplifier's had a go at it.
For the datatype, your use of field labels causes GHC to generate accessor functions. These aren't covered by your tests. Therefore the datatype shows as not completely covered. HTH, Andres -- Andres Löh, Haskell Consultant Well-Typed LLP, http://www.well-typed.com