Data.List.maximumBy uses counter-intuitive ordering

Carter Schonwald carter.schonwald at
Fri Jan 4 15:40:35 UTC 2019

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

On Thu, Jan 3, 2019 at 3:50 PM Ryan Reich <ryan.reich at> 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 <emertens at wrote:
>> 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
>> > On Dec 28, 2018, at 7:18 AM, Johannes Waldmann <
>> johannes.waldmann at> wrote:
>> >
>> > Dear all,
>> >
>> > this was brought up on the GHC tracker (not by me)
>> >
>> >
>> >
>> > 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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