Re: The purpose of LUA_ENVIRONINDEX pseudo index

[Jose Luis Hidalgo Valiño <joseluis.hidalgo@...>]

Wow!!!, thank you for the explanation now I have a new understanding  
about lua stack and its environments. Definitely the purpose of  
pseudo-indices is not very clear in the manual. I thought that  
objects at pseudo-indices were special objects that can be accessed  
but can not be changed because they are not really in the current  
stack. I was completely wrong.

That means , for example,  a function can change the global thread  
environment or even the registry with lua_replace(L,  
LUA_GLOBALSINDEX, ) or lua_replace(L, LUA_REGISTRYINDEX, ) , isn't  
it? Now I think about it, it  seems quite reasonable.

Jose L.

PD: And please excuse my bad English :)

El 29/08/2006, a las 18:29, Luis Carvalho escribió:

> On Tue, Aug 29, 2006 at 08:57:22AM +0200, Jose Luis Hidalgo Valiño  
> wrote:
> <snip>
> > LUA_ENVIRONINDEX seems a way to hold data locally to a set of
> > functions ( or only one ) but without loosing the possibility of
> > access the full global environment (the thread environment) through
> > LUA_GLOBALSINDEX.
>
> I like to think of LUA_ENVIRONINDEX as a sort of "lua_upvalueindex 
> (0)": it is
> like a shared upvalue among all C functions that have the same  
> environment. If
> by locally you mean private, then a closure is more appropriate, as  
> Mildred
> stated. Check toy example below.
>
> > In lua 5.1 lua_setfenv "changes" the behavior of LUA_ENVIRONINDEX or
> > LUA_GLOBALSINDEX ? I'm not sure.
>
> I don't understand what you mean, but since lua_setfenv changes the
> environment of a function, thread or userdata, it affects  
> LUA_ENVIRONINDEX for
> C functions.
>
> A simple example:
>
> /* environ.c */
> #include <lua.h>
> #include <lauxlib.h>
>
> static int environ_upv (lua_State *L) {
>   lua_Number e, u;
>   /* get environment data */
>   lua_rawgeti(L, LUA_ENVIRONINDEX, 1);
>   e = luaL_optnumber(L, -1, 0);
>   /* update environment */
>   lua_pushnumber(L, e+1);
>   lua_rawseti(L, LUA_ENVIRONINDEX, 1);
>   /* get closure data */
>   u = luaL_optnumber(L, lua_upvalueindex(1), 0);
>   /* update closure */
>   lua_pushnumber(L, u+1);
>   lua_replace(L, lua_upvalueindex(1));
>   lua_pushnumber(L, e);
>   lua_pushnumber(L, u);
>   return 2;
> }
>
> int luaopen_environ (lua_State *L) {
>   /* set current environment */
>   lua_createtable(L, 1, 0); /* environ table */
>   lua_pushnumber(L, 0);
>   lua_rawseti(L, -2, 1); /* environ[1] = 0 */
>   lua_replace(L, LUA_ENVIRONINDEX);
>   /* push closures with default environment (above) */
>   lua_pushnumber(L, 0);
>   lua_pushcclosure(L, environ_upv, 1);
>   lua_setfield(L, LUA_GLOBALSINDEX, "env1");
>   lua_pushnumber(L, 0);
>   lua_pushcclosure(L, environ_upv, 1);
>   lua_setfield(L, LUA_GLOBALSINDEX, "env2");
>   /* change environment using setfenv */
>   lua_pushnumber(L, 0);
>   lua_pushcclosure(L, environ_upv, 1);
>   lua_createtable(L, 1, 0); /* new environ table */
>   lua_pushnumber(L, 0);
>   lua_rawseti(L, -2, 1); /* environ[1] = 0 */
>   lua_setfenv(L, -2);
>   lua_setfield(L, LUA_GLOBALSINDEX, "env3");
>   return 0;
> }
>
> Lua 5.1  Copyright (C) 1994-2006 Lua.org, PUC-Rio
> > require'environ'
> > printupv = function(f) print(string.format('env: %d, upvalue: %d',  
> > f())) end
> > printupv(env1)
> env: 0, upvalue: 0
> > printupv(env1)
> env: 1, upvalue: 1
> > printupv(env2)
> env: 2, upvalue: 0
> > printupv(env2)
> env: 3, upvalue: 1
> > printupv(env3)
> env: 0, upvalue: 0
> > printupv(env3)
> env: 1, upvalue: 1
> > printupv(env1)
> env: 4, upvalue: 2
> > printupv(env1)
> env: 5, upvalue: 3
> > printupv(env2)
> env: 6, upvalue: 2
> > printupv(env2)
> env: 7, upvalue: 3
>
> For more examples of LUA_ENVIRONINDEX, check liolib.c.
>
> Cheers,
> Luis.
>
> --
> A mathematician is a device for turning coffee into theorems.
>         -- P. Erdos
>
> --
> Luis Carvalho
> Applied Math PhD Student - Brown University
> PGP Key: E820854A <carvalho@dam.brown.edu>