On Tue, Jul 26, 2011 at 7:46 PM, Evan Laforge
Could you give a specific example of the problem you're trying to solve?
Sorry, yes, that'd be useful :-) So, the project I'm working on involves developing a simulation of a securities market. I have a type which models an order book, on which orders can be placed or removed (and later filled): eg. placeOrder :: (Order e) -> e -> OrderBook -> OrderBook deleteOrder :: (Order e) -> e -> OrderBook -> OrderBook Now, i've chosen to model orders as a typeclass, as there are various semantic differences between different types of order that I can model as different types implementing this typeclass (limit orders vs market orders, buy side vs sell side), and these differences can be specified in the type's implementation of the class. However, there are a number of other typeclasses that all orders should also be instances of (and in terms of which their semantics don't differ, eg Eq or Ord. For instance, for a typeclass representing the interface that any Order type should implement: class Order o where price :: o -> Int size :: o -> Int I'd like to be able to specify an Eq instance for all types of class Order in a manner similar to this: instance (Order o) => Eq o where o1 == o2 = (price o1 == price o2) && (size o1 == size o2) I hope this clarifies my query - I'd be interested to know if this is possible, and whether or not it's a recommended approach, and if not, how else I could achieve something similar. Many thanks, Tim