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On 2018-02-11 11:55 AM, albertmcchan wrote:

does comma operator always process from left to right ? is it in the manual ?

items = 0
function count(...)
     items = items + select(''#', ...)
     return items
end

= count(1,2,3), items
3     3
= count(1,2,3), items
6     6

= items, count(1,2,3)
6     9
= items, count(1,2,3)
9     12





I don't think Lua specifies argument evaluation order, either. I can't 
find anything in the manual. Lua makes no guarantees about the execution 
of expressions, only statements.

(Aka x(y(), z()) is UB)


The only guarantee it seems to make is "Both `and` and `or` use 
short-circuit evaluation; that is, the second operand is evaluated only 
if necessary.":

http://www.lua.org/manual/5.3/manual.html#3.4.5

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