Re: [Haskell-cafe] Idiomatic ways to make all instances of a certain class also instances of another?

On Tue, Jul 26, 2011 at 1:52 PM, Tim Cowlishaw wrote:

On Tue, Jul 26, 2011 at 7:46 PM, Evan Laforge wrote:

...

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.

Use Maybe to demarcate nonsense semantics/undefinedness.

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.

data OrderType = Market Size | Limit LimitPrice Expiration Size | Stop (Either Percent Price) newtype Sell = Sell OrderType newtype Buy = Buy OrderType newtype Order = Order (Either Buy Sell) class Order o where

price :: o -> Int size :: o -> Int

size :: Order -> Int size (Order (Left (Buy (Market s))) = s size (Order (Left (Buy (Limit _ _ s))) = s etc.

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)