Re: table.sort: invalid order function for sorting

On Friday, April 17, 2026 at 8:55:59 AM UTC-4 Martin Eden wrote:

Hello guys, 

I've just found unobvious thing about table.sort: 

  -- ! invalid order function for sorting 
  table.sort({ 1, 2, 3, 1 }, function(a, b) return (a <= b) end) 

Lua manual says: 

  The comp function must define a consistent order; more formally, the 
  function must define a strict weak order. (A weak order is similar to 
  a total order, but it can equate different elements for comparison 
  purposes.) 

and for me <= operator is weak order.


The usual definition of strict weak orders does not permit this. In 
particular the comparison function should be irreflexive (f(x,x) is always 
false), asymmetric (f(x,y) being true implies f(y,x) is false), transitive 
(f(x,y) being true and f(y,z) being true implies f(x,z) is true), and 
transitive for values that compare equal (all of f(x,y), f(y,x), f(y,z), 
and f(z,y) being false implies f(x,z) and f(z,x) are both false). The 
comparison function here is reflexive (ignoring NaN) and not asymmetric, so 
it is not valid.
 




Practically it means that table.sort() never passes same item 
to comparator and when writing comparisons explicitly, 
default is "false": 

local compare = 
  function(a, b) 
    if (a < b) then return true end 
    if (a > b) then return false end 

    return false 
  end


return a<b
 




This error is not stable for my use case with comparing tables. 
"return false" default eliminates it but sometimes "return true" 
works too. 

Bug?


I do not see a bug here. Though it will not always diagnose a bad ordering.
 




-- Martin 

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