Re: Why can't I do for...in on a table?

[Wim Couwenberg <wcou@...>]

>     local t = { a = 1, b = "hello" }
>     for p in t do
>         print("t[",p,"] = ",t[p])
>     end
>
> This works in 5.0 and I use it extensively. 

This is legacy support in 5.0 and has been discussed at length.  In 5.1 
the support is dropped.  The official construction is

    for <vars> in <expression> do ... end

where expression is supposed to return three values:

    function, data, key

> nice to me, much nicer than table.foreach() or using pairs(). I know 
> that pairs has the nice feature that it gives me back the already 
> dereffed value. 

The function pairs basically does just this:

    function pairs(table) return next, table, nil end

so it uses next as its generator function.  Lua 4 did that as well (but 
in the same call frame).

> Also, it seems like it would be more efficient to use a table directly 
> for a couple of reasons. First, the special code that must have been in 
> place to handle a table directly would be faster than having to make a 
> function call every time.

Lua 4 didn't reenter the VM at least.  I suppose that was a bit faster. 
 Of course the VM of 5.0 is a fair bit faster overall.

> And if I just wanted the property and not the
> value, pairs() will do a deref that I don't need. 

That doesn't cost you anything extra, the value is stored in the same 
table node as the key.  Besides, Lua 4 did the same.

> What problem was being caused by "for p in t do" ???

No problem really.  I guess the new "generator" approach is found to be 
generally more attractive (string.gmatch is an example).  I know that 
not everyone agrees.

--
Wim