python: pattern matching

book.tex

@@ -16710,9 +16710,7 @@ class InterpLdyn(InterpLlambda):
 
   def untag(self, v, expected_tag, ast):
       match v:
-        case Tagged(val, tag):
-          if tag != expected_tag:
-            raise Exception('expected tag ' + expected_tag + ', not ' + ' ' + repr(v))
+        case Tagged(val, tag) if tag == expected_tag:
           return val
         case _:
           raise Exception('expected Tagged value with ' + expected_tag + ', not ' + ' ' + repr(v))