* In the ‘?’ operator, allow attribute paths. For instance, you can

write ‘attrs ? a.b’ to test whether ‘attrs’ has an attribute ‘a’
  containing an attribute ‘b’.  This is more convenient than ‘attrs ?
  a && attrs.a ? b’.

  Slight change in the semantics: it's no longer an error if the
  left-hand side of ‘?’ is not an attribute set.  In that case it just
  returns false.  So, ‘null ? foo’ no longer throws an error.

src/libexpr/nixexpr.cc

@@ -48,7 +48,7 @@ void ExprSelect::show(std::ostream & str)
 
 void ExprOpHasAttr::show(std::ostream & str)
 {
-    str << "(" << *e << ") ? " << name;
+    str << "(" << *e << ") ? " << showAttrPath(attrPath);
 }
 
 void ExprAttrs::show(std::ostream & str)
@@ -140,6 +140,17 @@ std::ostream & operator << (std::ostream & str, const Pos & pos)
 }
 
 
+string showAttrPath(const AttrPath & attrPath)
+{
+    string s;
+    foreach (AttrPath::const_iterator, i, attrPath) {
+        if (!s.empty()) s += '.';
+        s += *i;
+    }
+    return s;
+}
+
+
 Pos noPos;