Re: [Haskell-cafe] Re: (flawed?) benchmark : sort

Denis Bueno wrote:

On Tue, Mar 11, 2008 at 4:01 AM, Adrian Hey wrote:

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and sorting is meant to be a permutation, so we happily have the situation where this has a correct answer: 2.

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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. So really I think the docs have this backwards. It's sortBy that implements a stable sort (assuming a suitably sane comparison function I guess) and apparently sort is whatever you get from (sortBy compare). But this is unduly restrictive on possible correct sort implementations IMO. Regards -- Adrian Hey