Hi Dimitri,
It is not clear to me which one of these should be used and, most importantly, which criteria should be used to choose between them.
One criteria should be what you want to achieve, how you want to operate with your data.
1) newtype Volume = Volume Double deriving (Show) newtype Price = Price Double deriving (Show)
data Order = Bid {price::Price, volume::Volume} | Ask {price::Price, volume::Volume} deriving (Show)
data OrderBook = OrderBook{ bids::[Order] , asks::[Order] } deriving (Show)
This represenation of an Order gives you the ability to put Bids and Asks e.g. into the same list, but if you always want to keep them separate, then there's no point to have an ADT in the first place.
2) newtype Volume = Volume Double deriving (Show) newtype Price = Price Double deriving (Show)
data Order = Order {price::Price, volume::Volume} deriving (Show)
newtype Bid = Bid Order deriving Show newtype Ask = Ask Order deriving Show
data OrderBook = OrderBook{ bids::[Bid] , asks::[Ask] } deriving (Show)
If you want to keep Bids and Asks always separate, then this represenation seems to be more fitting.
3) newtype Volume = Volume Double deriving (Show) newtype Price = Price Double deriving (Show)
class Order a where makeOrder :: String -> String -> a
data Ask = Ask {aprice::Price, avolume::Volume} deriving (Show) instance Order Ask where makeOrder = makeAsk
data Bid = Bid {bprice::Price, bvolume::Volume} deriving (Show) instance Order Bid where makeOrder = makeBid
data OrderBook = OrderBook{ bids::[Bid] , asks::[Ask] } deriving (Show)
I might prefer the Asks and Bids data types of 2), because factoring out the common parts might have future benefits, by being able to reuse functionality on the common parts. The type class here fulfills something completely different to the previous two examples and is about the creation of your Asks and Bids. I don't think that you really need this type class, but you could just use makeAsk and makeBid without losing anything, because as long as your types in OrderBook are fixed you gain nothing. Greetings, Daniel