nil as lvalue (was Re: Proposal: Trailing comma in function calls)

[Tom N Harris <telliamed@...>]

On Thursday, June 05, 2014 09:50:30 AM Axel Kittenberger wrote:
> > Alternately allow `nil` as a lvalue that acts as the bit bucket.
> 
> While I'd appreciate something like that can we please keep some focus on
> the original proposal? And you know the rule, make a proposal, provide a
> patch.

Actually, I can't say I know that rule. But ignorance of the law is not an 
excuse.

In any case, I hacked a bit and got this to (seemingly) work

    a,nil,c = foo()

nil in a `local` declaration list is going to take a while longer because of 
the optimization that occurs. And I haven't looked at `for` yet. But this is 
what I'm aiming towards:

    stat ::= varlist '=' explist |
             local varnamelist ['=' explist] |
             for varnamelist in explist do block end

    varlist ::= var {',' var}

    var ::= Name | prefixexp '[' exp ']' | prefixexp '.' Name | nil

    varnamelist ::= varname {',' varname}

    varname ::= Name | nil

And the patch right now (against Lua 5.3work2):

diff --git a/src/lcode.c b/src/lcode.c
index 2e8b3fb..6d719ee 100644
--- a/src/lcode.c
+++ b/src/lcode.c
@@ -613,6 +613,10 @@ luaK_storevar
       luaK_codeABC(fs, op, var->u.ind.t, var->u.ind.idx, e);
       break;
     }
+    case VNIL: {
+      freeexp(fs, ex);
+      return;
+    }
     default: {
       lua_assert(0);  /* invalid var kind to store */
       break;
diff --git a/src/lparser.c b/src/lparser.c
index 9349149..cec3198 100644
--- a/src/lparser.c
+++ b/src/lparser.c
@@ -871,6 +871,11 @@ primaryexp
       luaK_dischargevars(ls->fs, v);
       return;
     }
+    case TK_NIL: {
+      init_exp(v, VNIL, 0);
+      luaX_next(ls);
+      return;
+    }
     case TK_NAME: {
       singlevar(ls, v);
       return;
@@ -1138,7 +1143,7 @@
 
 static void assignment (LexState *ls, struct LHS_assign *lh, int nvars) {
   expdesc e;
-  check_condition(ls, vkisvar(lh->v.k), "syntax error");
+  check_condition(ls, (vkisvar(lh->v.k) | (lh->v.k==VNIL)), "syntax error");
   if (testnext(ls, ',')) {  /* assignment -> ',' suffixedexp assignment */
     struct LHS_assign nv;
     nv.prev = lh;

-- 
tom <telliamed@whoopdedo.org>