Proposal: Make minimumBy and maximumBy go through foldl1

David Feuer david.feuer at
Thu Jan 26 02:34:26 UTC 2017

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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