Kein-Hong Man wrote:

`It is not heavy-duty constant folding, though. According to my
``observations, the terms of an expression are not reordered, so the
``following is not optimized:
`
local a = a+1+2
since it is parsed as:
local a = (a+1)+2

It would violate the language semantics to optimize this at
compile time. '+' is left-associative in Lua. If 'a' is a table
or userdata with an __add metamethod, it could completely change
the execution path when the terms are reordered or folded.
It even makes a difference with numbers (lua_Number = double):
$ lua -e 'local x=2^53; print((x+1+1)-x, (x+2)-x)'
0 2
Addition is only associative in specific (mathematical) groups.
E.g. for numbers with arbitrary precision or fixed precision
integers with wrap-around semantics. Floating-point numbers with
a fixed precision do not follow this rule (and many others).
Compiler writers sometimes forget about this (hint: check your
favourite language). But Lua gets it right.