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

[Rena <hyperhacker@...>]

On 2012-09-15 11:06 PM, "Greg Falcon" <veloso@verylowsodium.com> wrote:
>
> On Fri, Sep 14, 2012 at 7:20 AM, Roberto Ierusalimschy <roberto@inf.puc-rio.br> wrote:
>>
>> > > Why this simple code
>> > >
>> > > for i = 1, 5 do print(i) end
>> > >
>> > > cannot be written in more general form ?
>> >
>> > The only reasons I can think of are implementation optimization, and "a
>> > heritage" - everyone is used to this kind of for syntax.
>>
>> Other reasons include error detection. With the current syntax, Lua
>> can check whether the 'for' actually has two or three values, and
>> add the default 1 increment when it has only two. In the generic
>> form, Lua will have to adjust the list to three results.
>
>
> Why would it have to do so?  There is a difference, after all, between an _expression_ resulting in the two values <1, 10> and an _expression_ resulting in the three values <1, 10, nil>.
>
> Couldn't the runtime check the number of values resulting from the _expression_ before adjusting?  In principle, it seems you could enforce "step must be a number" only when the _expression_ resulted in 3 or more values, before adjustments.  This would give the generality being asked for here, without losing the error checking.
>
> Greg F

And then we get into the all too common "why is nil different from no value" discussion...