[gc_heap] Fix List::index() method to take negative numbers

examples/loops now works.

Also test out global container constants.

mycpp/examples/containers.py

@@ -10,11 +10,11 @@
 from typing import List, Tuple, Dict, Optional
 
 
-mystr = 'foo'  # type: str
-mylist = [1, 2]  # type: List[int]
-mylist2 = ['spam', 'eggs']  # type: List[str]
+gstr = 'foo'  # type: str
+glist_int = [1, 2]  # type: List[int]
+glist_str = ['spam', 'eggs']  # type: List[str]
 
-#mydict = {'a': 42, 'b': 43}  # type: Dict[str, int]
+gdict = {'a': 42, 'b': 43}  # type: Dict[str, int]
 
 
 def ListDemo():
@@ -103,6 +103,8 @@ def TupleDemo():
   else:
     print('no')
 
+  log("glist_int = %d", len(glist_int))
+  log("glist_str = %d", len(glist_str))
 
 
 def DictDemo():
@@ -121,6 +123,9 @@ def DictDemo():
     del d['foo']
     log('len(d) = %d', len(d))
 
+  # TODO: fix this
+  # log("gdict = %d", len(gdict))
+
 
 def run_tests():
   # type: () -> None

mycpp/examples/loops.py

@@ -27,6 +27,11 @@ def TestListComp():
   log('--- list comprehension changing type')
 
   z = ['[%d]' % i for i in x[1:-1]]
+
+  # I think this rewrite might be necessary?
+  #tmp1 = x[1:-1]
+  #z = ['[%d]' % i for i in tmp1]
+
   if mylib.PYTHON:
     #log("z = %s", z)
     pass

mycpp/gc_heap.h

@@ -796,12 +796,15 @@ class List : public gc_heap::Obj {
 
   // Implements L[i]
   T index(int i) {
+    if (i < 0) {
+      i += len_;
+    }
     if (i < len_) {
       return slab_->items_[i];
-    } else {
-      log("i = %d, len_ = %d", i, len_);
-      assert(0);  // Out of bounds
     }
+
+    log("i = %d, len_ = %d", i, len_);
+    assert(0);  // Out of bounds
   }
 
   // Implements L[i] = item