Re: Data.List.maximumBy uses counter-intuitive ordering

Does the inclusion of semi groups in base give another option? ? There’s perfectly nice left and right biased min and max semigroups. Though I think we only provide one flavor each for min and max. Either it is useful to document the behavior as an artifact of current implementation but not commit to the exact same semantic in future versions so as not to preclude future innovations ? One fuzzy thought: it almost seems like there’s two semigroup structures in play here: the aggregation / aka min or max in this case, plus implicitly a second structure that provides the tie breaking structure in the case of equality , which is usually a left or right bias, but could be some other mechanism. Is there a useful notion of a semigroup transformer or the like? On Thu, Jan 3, 2019 at 3:50 PM Ryan Reich wrote:

I think this is the right way to go: undefined when the maximum is duplicated. It is not implausible, for instance, that someone's mental model or even an alternate implementation of this function would, say, use the quicksort-like binary search for the maximum, and that really could give any of the options because quicksort is not stable.

There is no right answer to this question and therefore no answer should be demanded unless there is a compelling reason of efficiency.

Definitely document this, though.

On Fri, Dec 28, 2018, 07:27 Eric Mertens

...

Hello,

My opinion on this issue is that code should not be relying on the ordering of the choice made by maximumBy or minimumBy.

If we changed something I’d prefer to document that it is undefined what element is chosen when two are considered equal by the comparison function.

Code that relies on a particular earlier or later bias should use a function that makes it clear in the name that that’s what it’s doing. Readers should not be required to memorize the behavior of minimumBy or maximumBy in this regard to understand the code they are reading.

Best regards, Eric

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On Dec 28, 2018, at 7:18 AM, Johannes Waldmann < johannes.waldmann@htwk-leipzig.de> wrote:

Dear all,

this was brought up on the GHC tracker (not by me)

https://ghc.haskell.org/trac/ghc/ticket/15921

and it was suggested for discussion here.

my summary: Data.List.maximumBy is right-biased, minimumBy is left-biased, and none of this is documented.

- J.W.

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