[Haskell-cafe] Type classes

[Geest, G. van den]

I suppose you want to define compareEntries like this:
> compareEntries (Entry x) (Entry y) = compareEntries x y

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

> class DatabaseEntry e where
>    entryLabel     :: e -> String
>    formatEntry    :: e -> String
>    compareEntries :: e -> e -> Ordering
>
> data Entry a = Entry a
> 
> instance DatabaseEntry a => DatabaseEntry (Entry a) where
>    entryLabel (Entry e) = entryLabel e
>    formatEntry (Entry e) = formatEntry e
>    compareEntries (Entry x) (Entry y) = compareEntries x y



Gerrit


-----Original Message-----
From: haskell-cafe-bounces at haskell.org on behalf of Max Vasin
Sent: Mon 3/20/2006 3:46 PM
To: haskell-cafe at haskell.org
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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