Re: table.sort: invalid order function for sorting

On 2026-04-24 17:28, 'Lars Müller' via lua-l wrote:
> You can easily invert sorting. You just swap the two arguments:
>
> table.sort(Orders, function(a, b) return DateIsLess(b, a) end)

That's right. Came to that after 40 seconds since I sent email.

But we can't say it's obvious, right? Besides, in real life there will
be several lines of code instead of "a, b".

> Also not sure I get your point about needing to implement both "less 
> than" and "is equal".
> "Less than" suffices, you can default "is equal" to "not (lt(a, b) or 
> lt(b, a))".
>
> Needing two "comparator" calls to decide equality may of course not be 
> ideal for runtime,
> but tends to be fine since it usually just adds a constant factor of 
> about ~2 at worst.

Time spent in comparator is not limited. So sorting time may consist
mostly of time in that function. Imagine we're doing web request for
each comparison.. Imagine two times longer line of people in every
shop you visit..

> I think having comparators be boolean "less than" functions makes some 
> sense though.
> I want to be able to just write something like table.sort(t, 
> function(a, b) return a > b end).
>
> Introducing a kind of "spaceship operator" seems like too much for a 
> simple scripting language like Lua.
> What would the type of the result be? You could make it a number, of 
> course, but that's really a hack.
> (And one that tends to produce bugs when you have integer wraparound, 
> which Lua does.)
> I prefer the boolean-typed result of a "less than" comparison.
> (I contemplated "lt(a, b)" evaluating to nil if they a == b as a 
> convention for comparators,
> which would mostly still keep things working as nil is falsey, but 
> that's also hacky.)

Yeah it's hacky. But I agree that we need some three-ways binary 
operator for
scalar comparisons.

Boolean "less than" is perfectly fine. Problem that comparisons are costly.
So we spent like 24800 microseconds just to get data. And then
compare numbers and return 0 or 1. Which takes like 20 micros.

Ideally comparator should provide as much information about relation of
given items as it can. At least three output values if there is linear 
order.
What sorting algorithm will do with them is his own business.

>
> - Lars

-- Martin

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