# What does "hidden_function keyword" mean?

Hello, everyone. I am very new to Sage, so I am still getting used to it and learning the terminology. Recently, I was trying to test Sage out and I entered `vector(matrix([[]]))`

into the Sage terminal prompt, but it gave me an error message I did not understand:

```
sage: vector(matrix([[]]))
sage: vector(matrix([[]]))
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
<ipython-input-6-9573ec2e6342> in <module>()
----> 1 vector(matrix([[]]))
sage/modules/free_module_element.pyx in sage.modules.free_module_element.vector (build/cythonized/sage/modules/free_module_element.c:4462)()
/usr/lib/python2.7/dist-packages/sage/categories/finite_dimensional_modules_with_basis.pyc in _vector_(self, order)
385 # We slightly break encapsulation for speed reasons
386 return dense_free_module.element_class(dense_free_module,
--> 387 self.dense_coefficient_list(order),
388 coerce=True, copy=False)
389
/usr/lib/python2.7/dist-packages/sage/categories/finite_dimensional_modules_with_basis.pyc in dense_coefficient_list(self, order)
364 if order is None:
365 try:
--> 366 order = sorted(self.parent().basis().keys())
367 except AttributeError: # Not a family, assume it is list-like
368 order = range(self.parent().dimension())
/usr/lib/python2.7/dist-packages/sage/matrix/matrix_space.pyc in basis(self)
1187 return Family(keys, v.__getitem__,
1188 hidden_keys=list(range(self.dimension())),
-> 1189 hidden_function=old_index)
1190
1191 def dimension(self):
/usr/lib/python2.7/dist-packages/sage/sets/family.pyc in Family(indices, function, hidden_keys, hidden_function, lazy, name)
379 if hidden_keys == []:
380 if hidden_function is not None:
--> 381 raise ValueError("hidden_function keyword only makes sense "
382 "together with hidden_keys keyword !")
383 if function is None:
ValueError: hidden_function keyword only makes sense together with hidden_keys keyword !
```

When I originally ran this line of code, I was expecting it to just output the empty vector, $()$. However, instead, it gave me this cryptic error message about `hidden_function`

and `hidden_keys`

. Can someone please explain to me what this error message means? I can't find anything about this error message on Google. Thanks!

Hi Noble Mushtak

OS and SageMath version ?

for me :OS Ubuntu 18.04 SageMath 9.1

When I enter

`vector(matrix([[]]))`

, I got(), its ok for meI am using Ubuntu 18.04, Sage 8.1.