[Haskell-cafe] Type classes

[Matthias Fischmann]

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 at haskell.org
> From: Max Vasin <max-appolo at mail.ru>
> 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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