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

[Bart Massey]

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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