Re: luaffi vs luajit ffi

[James McKaskill <james@...>]

On Tue, May 22, 2012 at 12:03 PM, Cosmin Apreutesei
<cosmin.apreutesei@gmail.com> wrote:
> I'm using luajit ffi for a month now. I found that it's not just a
> library as it extends the language in some key semantics:
>
> (1) values of different types can be compared (Lua can't do that);
> this makes things easier most of the time, and a tricky sometimes[1]

There's really only two overloads that can't be handled:
Ptr vs nil
[u]int64_t vs number

The problem is that lua will check the types before handing off to the
metamethods. Lua 5.1 was worse in that it would never look up
metamethods for mixed types. Lua 5.2 improves things by looking up for
arith ops, but not for comparison ops IIRC.

> (2) type(cdata) == "cdata" -- so now we have the 9th Lua type -- ok
> you can overload type(), but maybe ffi.type() similar to io.type()
> would have been better, dunno.

type and tonumber are/will be overloaded. Thanks for mentioning type
btw I have added it to my todo list.

Other items:
LL ULL, and i extensions to the parser to generate int64_t, uint64_t,
and complex respectively.

> I don't think you can make luaffi a drop-in replacement for lj ffi
> without patching Lua to address (1) am I right?

You can get pretty darn close. The main one people hit is the
comparison against nil for pointers. I added a ffi.C.NULL as a
replacement. On my todo list is to move and/or copy this to ffi.NULL
(or ffi.null) that way ffi.NULL would resolve to nil under luajit and
the same code can work under both libraries.

> [1] for a null pointer a, `a == nil` is true but `not a` is false and
> `a = a or default` is not the same for a null pointer a and for a nil
> value a.

Frankly this goes to show that the ptr == nil breaks lua semantics, but alas.

At the moment the bigger issue w.r.t using the same code under both
luaffi and luajit is the slight difference in semantics (e.g. casting,
etc). These are mostly just bugs and polish.

As always if you find any issues where a bit of code works under
luajit but not under luaffi, please submit an issue to github
(https://github.com/jmckaskill/luaffi/issues).

-- James