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
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