Backport of Lua 5.2 "in" scoping (was Re: Lua registry, environment, and threads.)

[Mike Pall <mikelu-1001@...>]

Roberto Ierusalimschy wrote:
> Yes. The semantics is exactly as Mark explained:
> 
>   in t do command end 
> 
> is translated to
> 
>   do
>     local _temp = t;
>     <command>
>   end
> 
> and all acesses to <global> inside command are changed to _temp.global.

Sadly we don't get development snapshots for Lua 5.2. So I've
created a backport patch against Lua 5.1.4 from the above
description. It's entirely unsupported of course.

The intention is to allow everyone to experiment with this
feature, evaluate it's usefulness and/or complain if it doesn't
match their expectations. ;-)

[But don't complain to me -- I didn't propose the feature.]

--Mike

--- lua-5.1.4/src/lparser.h	2007-12-27 14:02:25 +0100
+++ lua-5.1.4-in1/src/lparser.h	2010-01-08 16:20:00 +0100
@@ -70,6 +70,7 @@
   int np;  /* number of elements in `p' */
   short nlocvars;  /* number of elements in `locvars' */
   lu_byte nactvar;  /* number of active local variables */
+  lu_byte globalvar;  /* global scope variable or NO_REG */
   upvaldesc upvalues[LUAI_MAXUPVALUES];  /* upvalues */
   unsigned short actvar[LUAI_MAXVARS];  /* declared-variable stack */
 } FuncState;
--- lua-5.1.4/src/lparser.c	2007-12-28 16:32:23 +0100
+++ lua-5.1.4-in1/src/lparser.c	2010-01-08 16:20:00 +0100
@@ -248,8 +248,16 @@
 static void singlevar (LexState *ls, expdesc *var) {
   TString *varname = str_checkname(ls);
   FuncState *fs = ls->fs;
-  if (singlevaraux(fs, varname, var, 1) == VGLOBAL)
-    var->u.s.info = luaK_stringK(fs, varname);  /* info points to global name */
+  if (singlevaraux(fs, varname, var, 1) == VGLOBAL) {
+    if (fs->globalvar == NO_REG)
+      var->u.s.info = luaK_stringK(fs, varname);  /* info has global name */
+    else {
+      expdesc key;
+      var->u.s.info = fs->globalvar;
+      codestring(ls, &key, varname);
+      luaK_indexed(fs, var, &key);
+    }
+  }
 }
 
 
@@ -342,6 +350,7 @@
   fs->nlocvars = 0;
   fs->nactvar = 0;
   fs->bl = NULL;
+  fs->globalvar = NO_REG;
   f->source = ls->source;
   f->maxstacksize = 2;  /* registers 0/1 are always valid */
   fs->h = luaH_new(L, 0, 0);
@@ -1268,6 +1277,24 @@
 }
 
 
+static void instat (LexState *ls, int line) {
+  /* stat -> IN exp1 DO block END */
+  FuncState *fs = ls->fs;
+  int oldglobal = fs->globalvar;
+  BlockCnt bl;
+  enterblock(fs, &bl, 0);
+  new_localvarliteral(ls, "(env)", 0);
+  exp1(ls);
+  fs->globalvar = fs->nactvar;
+  adjustlocalvars(ls, 1);
+  checknext(ls, TK_DO);
+  chunk(ls);
+  check_match(ls, TK_END, TK_DO, line);
+  leaveblock(fs);
+  fs->globalvar = oldglobal;
+}
+
+
 static int statement (LexState *ls) {
   int line = ls->linenumber;  /* may be needed for error messages */
   switch (ls->t.token) {
@@ -1314,6 +1341,11 @@
       breakstat(ls);
       return 1;  /* must be last statement */
     }
+    case TK_IN: {  /* stat -> instat */
+      luaX_next(ls);  /* skip IN */
+      instat(ls, line);
+      return 0;
+    }
     default: {
       exprstat(ls);
       return 0;  /* to avoid warnings */
--- lua-5.1.4/src/loadlib.c	2008-08-06 15:29:28 +0200
+++ lua-5.1.4-in1/src/loadlib.c	2010-01-08 16:20:00 +0100
@@ -565,7 +565,7 @@
   lua_pushvalue(L, -1);
   setfenv(L);
   dooptions(L, loaded - 1);
-  return 0;
+  return 1;
 }