8.7. Logic

JSONiq logics support is based on two-valued logics: there is just true and false and nothing else.

Non-boolean operands get automatically converted to either true or false, or an error is raised. The boolean() function performs a manual conversion. The rules for conversion were designed in such a way that it feels "natural". Here they are:

An empty sequence is converted to false.
A singleton sequence of one null is converted to false.
A singleton sequence of one string is converted to true except the empty string which is converted to false.
A singleton sequence of one number is converted to true except zero or NaN which are converted to false.
Operand singleton sequences of any other item cannot be converted and an error is raised.
Operand sequences of more than one item cannot be converted and an error is raised.

Example 8.12. Conversion to booleans.

{
  "empty-sequence" : boolean(()),
  "null" : boolean(null),
  "non-empty-string" : boolean("foo"),
  "empty-string" : boolean(""),
  "zero" : boolean(0),
  "not-zero" : boolean(1e42)
},
null and "foo"

Results:

{
  "empty-sequence" : false,
  "null" : false,
  "non-empty-string" : true,
  "empty-string" : false,
  "zero" : false,
  "not-zero" : true
}
false

8.7.1. Propositional Logic

JSONiq supports the most famous three boolean operations: conjunction, disjunction, and negation. Negation has the highest precedence, then conjunction, then disjunction. Comparisons have a higher precedence than all logical operations. Parentheses can override.

Example 8.13. Logics with booleans.

true and ( true or not true ),
1 + 1 eq 2 or not 1 + 1 eq 3

Results:

true
true

A sequence with more than one item, or singleton objects and arrays cannot be converted to a boolean. An error is raised if it is attempted.

Unlike in C++ or Java, you cannot rely on the order of evaluation of the operands of a boolean operation. The following query may return true or may raise an error.

Example 8.14. Non-determinism in presence of errors.

true or (1 div 0)

Results:

true