At 04:50 PM 5/15/2001 +1200, Tom Pledger wrote:
| --- | class (MonadPlus (p s v)) => Parser p where | item :: p s v v | force :: p s v a -> p s v a | first :: p s v a -> p s v a | papply :: p s v a -> s -> [(a,s)] | ---
[...]
The `SuitableCollection' class is pretty hard to define, though. Either it constrains its members to be list-shaped, or it prevents you from reusing functions like `item'. Hmmm... I think I've just stumbled across your reason for treating Parser as a class.
When the input isn't list-shaped, is the activity still called parsing? Or is it a generalised fold (of the input type) and unfold (of the result type)?
Well, it looks like Justin's answer to my "Databases" thread gave me the clue. What I want is called "Multiple parameter classes". Okasaki's code for implementing sets uses this extension to make a Set class. It wouldn't compile with nhc98 either, so I tried Hugs, which does support it if extensions are enabled, and has, in its documentation, a very nice explanation of the tradeoffs that using this extension entails. So, what I really want is something like: class (MonadPlus (p s v)) => Parser p s v | s -> v where item :: p s v v force :: p s v a -> p s v a first :: p s v a -> p s v a papply :: p s v a -> s -> [(a,s)] I haven't had the time to play with this yet, but it sounds promising... In case anyone is interested in the Hugs documentation of the feature: http://www.cse.ogi.edu/PacSoft/projects/Hugs/pages/hugsman/exts.html#exts Salutaciones, JCAB --------------------------------------------------------------------- Juan Carlos "JCAB" Arevalo Baeza | http://www.roningames.com Senior Technology programmer | mailto:jcab@roningames.com Ronin Entertainment | ICQ: 10913692 (my opinions are only mine) JCAB's Rumblings: http://www.metro.net/jcab/Rumblings/html/index.html