Henning Thielemann wrote:
I wonder whether the Ord instance is a good choice as constraint for Data.Map, Data.Set and so on.
In the case where there are Ord instances they are a good choice. Adding another type class for this purpose has the disadvantage that you have to write class instances for them, for all existing types that have Ord instances; while trivial this still adds up to quite a few lines of boilerplate code. This has to be weighted against the added boilerplate code for cases where a total order can be defined but is rather artificial. Right now I think that this is the exception rather than the rule. Changing the library interface should also not be done lightly. [snip examples: Complex numbers and Haskore notes]
I have a work-around in mind: I could introduce
class Eq a => Indexable a where compareIndex :: a -> a -> Ordering
[snip] Personally I'd do it without an Indexable class; just
newtype Index a = Index a
instance Ord x => Ord (Index (Complex x)) where (Index (Complex a b)) `compare` (Index (Complex c d)) = (a, b) `compare` (c, d)
should be good enough. 'Index' marks the purpose for the type. Note that for me, Ord alone really only implies a total order. If a type has both Num and Ord instances that is different; I understand your concerns about providing an Ord instance for Complex numbers directly.
Then I do not insert complex numbers immediately in a set, but wrap them in IndexableToOrd and insert the wrapped values in a set. Cumbersome, isn't it?
Of course you still need that index wrapping. If this gets out of hand it can be put into a simple wrapper around Data.Map though, without changing any existing libraries.
So my question is: Was the Ord constraint for Data.Map a good idea?
I still think it was. Bertram