Re: Abolishing the numeric-for

[Roberto Ierusalimschy <roberto@...>]

> On Mon, May 24, 2010 at 7:33 PM, M Joonas Pihlaja
> <jpihlaja@cc.helsinki.fi> wrote:
> > Regarding speed of numeric for and setup overhead of your "for i in
> > 100,1,1 do" style loop, you asked if the compiler couldn't optimise
> > the latter into the equivalent of a numeric for.  The answer to that
> > is probably "no, it can't (without significant effort)" because the
> > user can change the metatable of numbers whenever they please to make
> > the loop go up, down or sideways.
> 
> Only if you say so; the VM already ignores metamethods in several
> situations that involve primitive values (change __add for numbers all
> you want, addition is addition, for example). It would not be a
> stretch for numbers and functions to have hardcoded behaviors in this
> "for varlist in explist do" proposal.
> 
> I will try to summarize what I think is being proposed here:
> 
> -- first value of explist is a number
> for var in num, [num, [num]] do
>   -- same as current for var = num, [num, [num]] do
> end
> 
> -- first value of explist is a function
> for var, ... in func, ... do
>   -- same as the current generic for
> end
> 
> -- other cases
> for var, ... in val, ... do
>   -- do generic for using the result of
>   -- getmetamethod(val, "__iter")(...) as the explist
> end

Maybe I am missing something here, but way before the problem of
changing metamethods there is the problem of finding, at compile time,
whether a for loop is numerical, unless it uses literal numbers.
Syntactically, the three previous cases are all equal.

-- Roberto