Max,
class DatabaseEntry e where entryLabel :: e -> String formatEntry :: e -> String compareEntries :: e -> e -> Ordering
Then I define
data Entry = forall a. (DatabaseEntry a) => Entry a
instance DatabaseEntry Entry where entryLabel (Entry e) = entryLabel e formatEntry (Entry e) = formatEntry e
How can I define compareEntries for this instance?
In general: you can't. The field of the Entry constructor has a existentially quantified typed. Given two arbitrary values of type Entry, this type may be instantiated with a different type for each value, so you cannot easily compare the fields. If you extend the DatabaseEntry class such that it supplies a method that allows to produce some canonical representation for database entries suited for comparison, then you could take that road. Are you sure that your Entry type needs to be existentially quantified? HTH, Stefan