Re: Unordered map

Andrew Wagner wrote:

Suppose I have some data structure that is inherently unordered, and I want to use it as a key into a Map. How would I get around this limitation on the client side? Well, the most obvious way is to create some nonsensical Ord instance, like compare _ _ = Eq. Is that the recommended solution?

instance Ord ... where compare _ _ = EQ is a bad idea, because it makes all elements equal and therefore all maps will be empty or singletons. (In order to construct such maps the order is not needed.) The recommended solution is to put "deriving (Eq, Ord)" after your data types for keys. (At least an Eq instance is needed for association lists, i.e. "Prelude.lookup") Cheers Christian