Re: [Haskell-cafe] Re: Type classes

Then you should produce 'some canonical representation for database entries suited for comparison', like Stefan mentioned. For example:

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

instance DatabaseEntry Entry where entryLabel (Entry e) = entryLabel e formatEntry (Entry e) = formatEntry e compareEntries (Entry x) (Entry y) = compare (entryLabel x) (entryLabel y)

Gerrit Max Vasin wrote:

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"Geest," == Geest, G van den writes:

Geest,> I suppose you want to define compareEntries like this:

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compareEntries (Entry x) (Entry y) = compareEntries x y

Geest,> An option is to just implement it the following way Geest,> (Haskell98!):

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

data Entry a = Entry a

No. I don't want that. The database parsing function returns Map.Map String Entry but entries can of different types (and these type vary over styles).