Re: Assignment statement does not evaluate all its expressions first

[Xmilia Hermit <xmilia.hermit@...>]

Francisco Olarte:

>  From what I see:
>
>     stat ::= varlist ‘=’ explist
>     varlist ::= var {‘,’ var}
>     explist ::= exp {‘,’ exp}
>
> It looks like the "expressions" refer only to the RHS ones. Somewhere
> above vars are defined using prefixexp and exp , and a comment stating
> that may be clarifying.
> I think it may be a lack of detail. The manual, IMO, is trying to say that
>
> up.x,up.y=up.y,up.x
>
> works somehow like a=up.y, b=up.x, up.x=a, up.y=b, which may not be
> obvious. In fact your example, from what I read, is behaving correctly
> with all the exp, and is hitting the var[exp] variant of prefixexp
> correctly preevaluating the exp part, but failing on the var part.
So
node.next, node = new, new
could also not evaluate the node from node.next before node is assigned 
to new, however, this case is explicitly checked in 
https://github.com/lua/lua/blob/9db4bfed6bb9d5828c99c0f24749eedf54d70cc2/lparser.c#L1330. 
Why even do this check in the first place if the prefix-expression parts 
do not need to be evaluated before the assignments?
Further, from https://www.lua.org/manual/5.4/manual.html#3.4 I would 
assume that prefixexp are also expressions.
And if by expressions only the RHS is meant the example:

Thus the code

i = 3
i, a[i] = i+1, 20
sets a[3] to 20, without affecting a[4] because the i in a[i] is 
evaluated (to 3) before it is assigned 4.

would be wrong, as i could be assinged 4 before a[i] is evaluated.

It would be nice to know how this is meant to work as the my example is 
a bit contrived, however, this could happen in a more complex __newindex 
metamethod which for example calls to other hook functions.

Regards,
Xmilia