[Haskell-cafe] Re: Type classes

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"Stefan" == Stefan Holdermans writes:

Stefan> Max,

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class DatabaseEntry e where entryLabel :: e -> String formatEntry :: e -> String compareEntries :: e -> e -> Ordering Then I define

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data Entry = forall a. (DatabaseEntry a) => Entry a

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instance DatabaseEntry Entry where entryLabel (Entry e) = entryLabel e formatEntry (Entry e) = formatEntry e How can I define compareEntries for this instance?

Stefan> In general: you can't. The field of the Entry constructor has Stefan> a existentially quantified typed. Given two arbitrary values Stefan> of type Entry, this type may be instantiated with a different Stefan> type for each value, so you cannot easily compare the fields. Yes, this require something like multimethods in CLOS. Stefan> If you extend the DatabaseEntry class such that it supplies a Stefan> method that allows to produce some canonical representation Stefan> for database entries suited for comparison, then you could Stefan> take that road. Stefan> Are you sure that your Entry type needs to be existentially Stefan> quantified? I want a map of entries (mapping lables to entries). Generally each style can define its own data types for database objects. Converting entries from data BibEntry = { beLabel :: String , beKind :: String , beProperties :: Map.Map String String } is some sort of "static type checking" of the database. -- WBR, Max Vasin.