move error check before return in map_product

pymbolic/geometric_algebra/mapper.py

@@ -270,6 +270,11 @@ def map_product(self, expr):
                 nabla_finder.setdefault(
                         ncomp.nabla_id, set()).add((child_idx, ncomp.ambient_axis))
 
+        if nabla_finder and not any(d_source_nabla_ids_per_child):
+            raise ValueError(f"no derivative source found to resolve in '{expr}'"
+                    " -- did you forget to wrap the term that should have its "
+                    "derivative taken in 'Derivative()(term)'?")
+
         if not has_d_source_nablas:
             rec_children = [self.rec(child) for child in expr.children]
             if all(rec_child is child
@@ -280,11 +285,6 @@ def map_product(self, expr):
 
         # }}}
 
-        if nabla_finder and not any(d_source_nabla_ids_per_child):
-            raise ValueError(f"no derivative source found to resolve in '{expr}'"
-                    " -- did you forget to wrap the term that should have its "
-                    "derivative taken in 'Derivative()(term)'?")
-
         # a list of lists, the outer level presenting a sum, the inner a product
         result = [list(expr.children)]