On 10 Mar 2008, at 4:00 AM, Adrian Hey wrote:
Neil Mitchell wrote:
2) What does it do with duplicate elements in the list? I expect it deletes them. To avoid this, you'd need to use something like fromListWith, keeping track of how many duplicates there are, and expanding at the end. That would be wrong. Consider: data Foo = Foo Int Int instance Ord Foo where compare (Foo a _) (Foo b _) = compare a b
I would consider such an Ord instance to be hopelessly broken, and I don't think it's the responsibility of authors of functions with Ord constraints in their sigs (such as sort) to consider such possibilities or specify and control the behaviour of their behaviour for such instances. Trying to do this is what leads to horrors such as the "left biasing" of Data.Map (for example).
Data.Map is implicitly using such an Ord instance behind the scenes, and I think it has to to maintain its own invariants. If I take the `union' of two maps that take the same key to different values, I have to decide which value to use, even if every Ord instance supplied by my clients is 100% Adrian-compliant. jcc