Re: Multiple values returned or vararg expression in numeric for loop

[Coda Highland <chighland@...>]

On Thu, Sep 13, 2012 at 4:47 PM, Hisham <hisham.hm@gmail.com> wrote:
> On Thu, Sep 13, 2012 at 6:51 PM, Dimitris Papavasiliou
> <dpapavas@gmail.com> wrote:
>> Egor does have a point though.  Since the for statement accepts a list
>> of values one would expect the usual rules about function calls and
>> adjustment of returned values to apply.  If you can say
>
> The usual rules do apply. Whenever a single value is expected and
> multiple values are returned, the extra values are discarded.
>
>> function f() return 2,10 end
>> function g() return 4,12 end
>> for a = f(), g() do print(a) end
> 2
> 3
> 4
>
> As Dirk said, the comma-separated values are not a "list of values" in
> the numeric "for" syntax. The fact that it uses "=" and "," just like
> assignments is a coincidence, just like "(" is used both to
> parenthesize sub-expressions and to start the list of arguments in a
> function: two different uses of the same character. The "a = 1, 2"
> inside "for a = 1, 2 do ... end" is not an assignment. The syntax
> might as well have been "for a from 1 to 2 do ... end".
>
> -- Hisham
> http://hisham.hm/
>

Certainly we understand the facts of how it's implemented and how the
syntax is structured. That's not the point, though. The point is that
it LOOKS like it ought to work. If Lua patterns were to hold, the
syntax would be "for <variable> = <exprlist> do" rather than "for
<variable> = <expr>, <expr>[, <expr>] do". Certainly there's valid
reasons for this, but it IS different, and that was what they were
trying to say.

(The function call example isn't a great one anyway, because in the
case of parenthesized expressions the ( comes BEFORE the variable
name, but in the case of a function call the ( comes AFTER it. They
look different.)

/s/ Adam