Proposal: make minimumBy/maximumBy go through foldl', not foldr1

[Sumit Sahrawat, Maths & Computing, IIT (BHU)]

foldl1 is lazy, whereas foldl1' is strict. The primes are usually used to
indicate strictness.

On 29 October 2015 at 23:59, Bart Massey <bart.massey at gmail.com> wrote:

> I'm super-confused here. It is claimed that the Haskell 98 Report requires
> List.minimumBy and List.maximumBy to be lazy, but the sample
> implementations given there use foldl1 (
> https://www.haskell.org/onlinereport/list.html) and I can't find anyplace
> in the report where that's explicitly said to be wrong. What am I missing?
>
> On Thu, Oct 29, 2015 at 11:15 AM David Feuer <david.feuer at gmail.com>
> wrote:
>
>> I think it would be very nice to offer a few folds tailored to specific
>> strictness patterns. I'm not sure of the best interface, however.
>> On Oct 29, 2015 2:07 PM, "Joachim Breitner" <mail at joachim-breitner.de>
>> wrote:
>>
>>> Hi,
>>>
>>> Am Donnerstag, den 29.10.2015, 17:34 +0200 schrieb Roman Cheplyaka:
>>> > I just realized (thanks to this reddit thread[1]) that minimumBy is now
>>> > implemented through foldr1. Previously (before FTP), minimumBy from
>>> Data.List
>>> > used foldl1, while minimumBy from Data.Foldable used foldr1.
>>> >
>>> > I think that the way these were merged was a mistake.
>>>
>>> this has also been raised by Neil at
>>> https://ghc.haskell.org/trac/ghc/ticket/10830
>>>
>>> Also note https://ghc.haskell.org/trac/ghc/ticket/3416 where it is
>>> stated that the report demands minimum (and hence minimumBy) to be
>>> lazy.
>>>
>>> Ignoring that argument, and that of not changing libraries light-
>>> harted, I do think that foldl1' would be the better choice here; users
>>> who rely on laziness here are likely in the minority and it would be
>>> reasonable to expect them to manually use foldl1 or foldr1.
>>>
>>> BTW, why is there no fold' method in Foldable that leaves the
>>> associativity (left, right, or mixed in case of a tree) to the Foldable
>>> instance?
>>>
>>>
>>> Greetings,
>>> Joachim
>>>
>>> --
>>> Joachim “nomeata” Breitner
>>>   mail at joachim-breitner.de • http://www.joachim-breitner.de/
>>>   Jabber: nomeata at joachim-breitner.de  • GPG-Key: 0xF0FBF51F
>>>   Debian Developer: nomeata at debian.org
>>>
>>>
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-- 
Regards

Sumit Sahrawat
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