Ticket #15305: coercion_for_tensor_product-nb.patch

sage/combinat/free_module.py

@@ -2536 +2536 @@
             """
             return self.tensor_constructor(tuple(element.parent() for element in elements))(*elements)
 
+        def _coerce_map_from_(self, TensorParent):
+            r"""
+            EXAMPLES::
+
+                sage: C = CombinatorialFreeModule(ZZ, Set([1,2]))  
+                sage: D = CombinatorialFreeModule(ZZ, Set([2,4]))  
+                sage: f = C.module_morphism(on_basis=lambda x: D.monomial(2*x), codomain=D)
+                sage: f.register_as_coercion()
+                sage: T = tensor((C,C))
+                sage: p = D.an_element()
+                sage: T(tensor((p,p)))
+                Traceback (most recent call last):
+                ...
+                AssertionError: do not know how to coerce Free module generated by {2, 4} over Integer Ring # Free module generated by {2, 4} over Integer Ring into Free module generated by {1, 2} over Integer Ring # Free module generated by {1, 2} over Integer Ring (there is no coercion map from Free module generated by {2, 4} over Integer Ring into Free module generated by {1, 2} over Integer Ring)
+                sage: T = tensor((D,D))
+                sage: p = C.an_element()
+                sage: T(tensor((p,p))) 
+                4*B[2] # B[2] + 4*B[2] # B[4] + 4*B[4] # B[2] + 4*B[4] # B[4]
+            """
+            assert(isinstance(TensorParent, CombinatorialFreeModule_Tensor))
+            modules = TensorParent._sets
+            assert(module in ModulesWithBasis(self.base_ring()) for module in modules)
+            for i, Parent_i in enumerate(modules):
+                assert (self._sets[i]).has_coerce_map_from(Parent_i), "do not know how to coerce %s into %s (there is no coercion map from %s into %s)"%(TensorParent, self, Parent_i, self._sets[i])
+            vector_map = [(self._sets[i]).coerce_map_from(module) for i,module in enumerate(modules)]
+            result = TensorParent.module_morphism(on_basis=lambda x: self._tensor_of_elements([vector_map[i](modules[i].monomial(x[i])) for i in range(len(modules))]), codomain=self)
+            return result
+
+
 class CartesianProductWithFlattening(object):
     """
     A class for cartesian product constructor, with partial flattening