Re: table.sort: invalid order function for sorting

Yes, I know that Lua went with the "<" convention, so "<=" is a problem.

Maybe I'm misunderstanding Philippe's point, but I don't see how "a <= 
b" would be a "good" comparator while "a < b" would be a "bad" 
comparator if one can easily be turned into the other by swapping 
arguments and inverting the result.

(Strictly speaking this equivalence is of course not true with 
"uncomparable" items like NaN, but (1) once again this is intentional, 
and (2) I don't see why <= would be preferable. And "uncomparability" 
can still be tested in the same way, by checking whether both a <= b 
and b <= a resp. a < b and b < a are false.)

- Lars


On Mon, Apr 27 2026 at 00:33:11 +02:00:00, 'Martin Eden' via lua-l 
<[email protected]> wrote:
> On 2026-04-26 21:32, 'Lars Müller' via lua-l wrote:
>> How so? There is no meaningful difference between "a < b" and "a <= 
>> b"  as convention for a comparator,
>> because "a <= b" is equivalent to "not (b < a)". It is just a 
>>  convention, and a pretty reasonable one at that.
>> 
>> - Lars That's the original issue of this thread.
> 
>>   Lua 5.3.6  Copyright (C) 1994-2020 Lua.org, PUC-Rio
>>   > table.sort({ 1, 2, 3, 1 }, function(a, b) return not (a > b) end)
>>   stdin:1: invalid order function for sorting
>>   stack traceback:
>>     [C]: in function 'table.sort'
>>     stdin:1: in main chunk
>>     [C]: in ?
> 
> You can't have (a <= b) comparator in table.sort().
> 
>> 
>> On Sun, Apr 26 2026 at 13:18:38 +02:00:00, Philippe Verdy 
>>  <[email protected] <mailto:[email protected]>> wrote:
>>> If you want stability, suitable for mitlkey sort, or sorts in 
>>>   multiple passes, and the minimum of swaps for duplicate values, 
>>> or   for NaN values), using the comparator (a<b) does not work at 
>>> all, but   (a<=b) does.
>>> 
>>> Lua made the worst decision for its binary comparator used in 
>>>   table.sort!
> Many recursive sorts are not stable. So you have to use multi-tier 
> comparator
> because you can't afford multiple passes as in BucketSort. I am more 
> saddened
> about random run-time explosive effect of this error. Your program may
> pass your tests but still explode in a wild.
> 
> -- Martin
> 
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