Use a negative stack index in examples/zig-fn.zig

Closes #114

examples/zig-fn.zig

@@ -22,8 +22,6 @@ pub fn main() anyerror!void {
     defer _ = gpa.deinit();
 
     // Initialize The Lua vm and get a reference to the main thread
-    //
-    // Passing a Zig allocator to the Lua state requires a stable pointer
     var lua = try Lua.init(allocator);
     defer lua.deinit();
 
@@ -42,8 +40,11 @@ pub fn main() anyerror!void {
     lua.protectedCall(.{ .args = 2, .results = 1 }) catch unreachable;
 
     // The result of the function call is on the stack.
-    // Use toInteger to read the integer at index 1
-    std.debug.print("the result: {}\n", .{lua.toInteger(1) catch unreachable});
+    // Use toInteger to read the integer at index -1.
+    // Using a negative stack offset will get the value relative to the top of the stack.
+    // Because nothing else is on the stack, index 1 would also work here. The negative index can
+    // be more reliable when the contents of the stack before the function call are unknown.
+    std.debug.print("the result: {}\n", .{lua.toInteger(-1) catch unreachable});
 
     // We can also register the function to a global and run from a Lua "program"
     lua.pushFunction(ziglua.wrap(adder));