Re: help to (slightly) modify Lua interpreter

[Mark Feldman <mfeldman@...>]

steve donovan wrote:
> This is in fact the case that makes this so non-trivial.  It is not
> difficult to do a token-filter that copes with the difference between
> A[i,j] and A[i,j] = value, but multiple assignment needs a proper AST
> transformation.
>   

I created a patch for multi-dimensional indexing over the weekend (see 
code below) and pretty-much reached the same conclusion. I eventually 
gave up on using multiple parameters for the indexing and instead 
modified the parser to treat m[1,2] the same as m[1][2]. Apart from 
being a very small patch (and faster to parse) it doesn't break multiple 
assignment or constructors. Of course, I personally would hope that this 
never officially makes it into Lua because it forces a very specific and 
unnessessary syntax onto the language IMHO, but for the time being it 
would probably be good enough for what Francesco is after.

As far as the technical implementation goes my patch does 3 things:

1) The yindex function (which parses table indices) has been modified so 
that it doesn't parse the outer '[' and ']' tokens. This is now handled 
by the calling functions.
2) The loop in primaryexp has been modified to keep looping and indexing 
while the ',' token is encountered.
3) The recfield function has had a couple of lines added so that parsing 
of constructors remains the same. Statements such as m={[1][2]="foo"} 
could technically be made to work but they're nonsensical and would fail 
since m[1] is nil to begin with. Also the existing parser doesn't allow 
statements such as m={[1]={}, [1][2]="foo"} so I've kept that behaviour 
the same.

I haven't tested this patch exhaustively but looking at the way the code 
works I think it should be ok, it seems to have no problem handling code 
like this:

-- test multidimensional arrays. also test constructors
-- and multiple assignment to show they're not broken
m = {[1]={}, [2]="foo"}
m[1,2] = "bar"
print(m[2], m[1][2], m[1,2])    --> foo  bar bar
m.foo = {}
m.foo.bar = "grok"
print(m["foo","bar"])           --> grok

Cheers,

Mark Feldman

=== mda.patch===
diff C:\Temp\lua-5.1.4\src/lparser.c lua-5.1.4\src/lparser.c
420d419
<   luaX_next(ls);  /* skip the '[' */
423d421
<   checknext(ls, ']');
453a452,453
>   {
>     luaX_next(ls);
454a455,456
>     checknext(ls, ']');
>   }
703,705c705,712
<         luaK_exp2anyreg(fs, v);
<         yindex(ls, &key);
<         luaK_indexed(fs, v, &key);
---
>         luaX_next(ls);
>         do
>         {
>             luaK_exp2anyreg(fs, v);
>             yindex(ls, &key);
>             luaK_indexed(fs, v, &key);
>         } while (testnext(ls, ','));
>         checknext(ls, ']');



This message and its attachments may contain legally privileged or confidential information. This message is intended for the use of the individual or entity to which it is addressed. If you are not the addressee indicated in this message, or the employee or agent responsible for delivering the message to the intended recipient, you may not copy or deliver this message or its attachments to anyone. Rather, you should permanently delete this message and its attachments and kindly notify the sender by reply e-mail. Any content of this message and its attachments, which does not relate to the official business of the sending company must be taken not to have been sent or endorsed by the sending company or any of its related entities. No warranty is made that the e-mail or attachment(s) are free from computer virus or other defect.