Chapter 18. Null vs. Empty Sequence

Null and the empty sequence are two different concepts.

Null is an item (an atomic value), and can be a member of an array or of a sequence, or the value associated with a key in an object. Empty sequences cannot, as they represent the absence of any item.

Example 18.1. Null values in an array

[ null, 1, null, 2 ]

Results:

[ null, 1, null, 2 ]

Example 18.2. Null values in an object

{ "foo" : null }

Results:

{
  "foo" : null
}

Example 18.3. Null values in a sequence

(null, 1, null, 2)

Results:

null
1
null
2

If an empty sequence is found as an object value, it is automatically converted to null.

Example 18.4. Automatic conversion to null.

{ "foo" : () }

Results:

{
  "foo" : null
}

In an arithmetic opration or a comparison, if an operand is an empty sequence, an empty sequence is returned. If an operand is a null, an error is raised except for equality and inequality.

Example 18.5. Empty sequence in an arithmetic operation.

() + 2

Results:

Example 18.6. Null in an arithmetic operation (failing).

null + 2

Error:

arithmetic operation not defined between types "js:null" an
d "xs:integer"

Example 18.7. Null and empty sequence in an arithmetic operation.

null + ()

Results:

Example 18.8. Empty sequence in a comparison.

() eq 2

Results:

Example 18.9. Null in a comparison.

null eq 2

Results:

false

Example 18.10. Null in a comparison.

null lt 2

Results:

true

Example 18.11. Null and the empty sequence in a comparison.

null eq ()

Results:

Example 18.12. Null and the empty sequence in a comparison.

null lt ()

Results: