scripts: csv.py: Improved default typechecking in RExpr

Now, by default, an error is raised if any branch of an expr has an
inconsistent type.

This isn't always what we want. The ternary operator, for example,
doesn't really care if the condition's type doesn't match the branch
arms. But it's a good default, and special cases can always override the
type function with their own explicit typechecking.

scripts/csv.py

@@ -346,7 +346,10 @@ class RExpr:
             return set(it.chain.from_iterable(v.fields() for v in self))
 
         def type(self, types={}):
-            return self.a.type(types)
+            t = self.a.type(types)
+            if not all(t == v.type(types) for v in it.islice(self, 1, None)):
+                raise RExpr.Error("mismatched types? %r" % self)
+            return t
 
         def fold(self, types={}):
             return self.a.fold(types)