Proposal: Make minimumBy and maximumBy go through foldl1

Joe Hillenbrand joehillen at
Thu Jan 26 03:31:55 UTC 2017

Would you care to elaborate?

On Wed, Jan 25, 2017 at 6:34 PM, David Feuer <david.feuer at> wrote:
> I doubt it very much, and some of us have serious doubts about the
> extension as it stands.
> On Wed, Jan 25, 2017 at 8:58 PM, Joe Hillenbrand <joehillen at> wrote:
>> Would the (hopeful) imminent linear types extension help here?
>> On Wed, Jan 25, 2017 at 4:24 PM, Edward Kmett <ekmett at> wrote:
>>> The current intent based on the ghc ticket traffic is to switch the
>>> implementation to foldl1 for the next release as a stopgap, but leave the
>>> door open to do something smarter in the future.
>>> The current foldr1 implementation is simply never a win, and a monoidal
>>> version devolves to a right fold for lists with the same bad behavior.
>>> If we later on figure out a way to efficiently exploit a strict monoidal
>>> accumulator for monoids that don't benefit from short-circuiting then a
>>> number of current Foldable combinators could benefit including this one, so
>>> we definitely want to leave the door open to doing things better in the
>>> future, but for now the left fold is an easy improvement over the status
>>> quo.
>>> -Edward
>>> On Wed, Jan 25, 2017 at 6:50 PM, David Laing <dave.laing.80 at>
>>> wrote:
>>>> Hi all,
>>>> I'm proposing that the implementations of minimumBy and maximumBy be
>>>> changed from using foldl1 to foldr1.
>>>> I found this in a GHC trac ticket[0] labelled 'Newcomer' that has more
>>>> details / discussion on that.
>>>> The points that stand out to me are:
>>>> - the Haskell report says that those methods should be implemented in
>>>> terms of foldl1 (although as instance methods of Foldable there might be
>>>> some wiggle room there)
>>>> - it helps solves a space leak (which at first glance feels like it might
>>>> be a more common problem than the options that foldl1 removes)
>>>> - from the discussion on the ticket, it seemed to be an agreeable middle
>>>> ground
>>>> As a side note: there have been a few proposals in the past to switch
>>>> these functions to use foldl', which seemed to have stalled.  I don't know
>>>> what the etiquette is around bringing up minor variations on old proposals
>>>> again, so I apologise if I've breached some kind of protocol here.
>>>> Although I guess I've already breached one protocol by pushing a  patch to
>>>> Phabricator for review without getting sign-off from the Core Libraries
>>>> Committee, so at least I'm being even handed with my clumsiness :)
>>>> Cheers,
>>>> Dave
>>>> [0]
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