[Haskell-cafe] Type classes

[Stefan Holdermans]

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