Design questions for a small s-expression library

[Antonio Vieiro <antonio@...>]

Hi all,

I would like to build a small s-expression ([1]) library in Lua (5.1), 
so I need to design cons-cells ([2]) first.

I would appreciate some advice in the design of the cons-cells. I am 
thinking of two approaches:

A) Design cons-cells as Lua tables, in plain Lua, with a "car" and a 
"cdr" being other Lua objects (possibly other cons-cells).
B) Design cons-cells as a C structure (Lua allocated and garbage 
collected with a __gc metamethod), that keeps references to other Lua 
objects using luaL_ref and luaL_unref (possibly with references to other 
cons-cells).

I am thinking that approach B is going to be more performant and consume 
less memory. These are my assumptions:

1. B is better because using a Lua table as a cons cell is too heavy 
memory-wise.
2. B Is better because I can abuse LUA_REGISTRYINDEX to store references 
to Lua objects, that's better than having as many Lua tables in memory.
3. LuaJIT will be as performant in approach B (invoking C functions for 
"car", "cdr", etc.) as in approach A (using Lua native code).
4. There's no problem in having circular dependencies using luaL_ref's 
between different cons cells, because the Lua GC will detect that and 
cleanse cons-cells appropriately.

Am I right in these assumptions? I'm a little worried about point 4.

Thanks in advance,
Antonio


[1] http://en.wikipedia.org/wiki/S-expressions
[2] http://en.wikipedia.org/wiki/Cons