[Haskell-cafe] Re: (flawed?) benchmark : sort
jules at jellybean.co.uk
Wed Mar 12 09:56:05 EDT 2008
Adrian Hey wrote:
> Denis Bueno wrote:
>> On Tue, Mar 11, 2008 at 4:01 AM, Adrian Hey <ahey at iee.org> wrote:
>>> > and sorting is
>>> > meant to be a permutation, so we happily have the situation where
>>> > has a correct answer: 2.
>>> > Anything else is incorrect.
>>> Isn't 3 also a permutation? Why is it incorrect?
>> Because it is not stable.
>> The documentation for Data.List.sort says the sort is stable:
>> "The sort function implements a stable sorting algorithm."
>> A stable sort respects the order of equal elements as they occur in
>> the input list.
> This might be a reasonable thing to say about *sortBy*, but not sort
> as the ordering of equal elements should not be observable (for any
> correct instance of Ord). It should be impossible to implement a
> function which can discriminate between [a,a],[a,b],[b,a],[b,b] if
> compare a b = EQ.
The fact that you can't implement a function to differentiation does not
meant the difference is not important.
"[b,a]" might cause 500G of file IO which "[a,b]" will not cause.
This is not observable within haskell, but is very observable to the user.
Stability is a nice property. I don't understand why you are arguing
against this so aggressiviely.
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