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

On Tue, Jul 26, 2011 at 11:14 PM, Alexander Solla wrote:

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)

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

Aah, thank you - this is really neat. So now, I can write (for instance) an Eq instance for OrderType and use deriving (Eq) on the newtypes that wrap it, and my Order can be a concrete type, but still encapsulates all the different types of order. Thank you! Tim Thank you