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

see my discussion a few moments ago, in particular my posting http://www.haskell.org/pipermail/haskell-cafe/2006-March/014981.html as you by now already know from this thread, the link tells you that the only possible solution is to turn the two entries to be compared into something of the same type, which can only be done with another type class. i am using 'Show' for now and compare the strings, because it's really simple and i don't care about performance at this stage of the project. might bite me later, though. cheers, matthias On Mon, Mar 20, 2006 at 05:46:43PM +0300, Max Vasin wrote:

To: haskell-cafe@haskell.org From: Max Vasin Date: Mon, 20 Mar 2006 17:46:43 +0300 Subject: [Haskell-cafe] Type classes

Hi!

I'm currently experimenting with a bibliography generation tool for LaTeX. It will (if it will be finished) use BibTeX databases but bibliography styles will be written in Haskell. I want styles to be able to transform database entries into some style specific data type, so I define

...

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?

-- WBR, Max Vasin.

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