Logics
JSONiq follows the W3C standard for logical expressions; it introduces a prefix unary not operator as a synonym for fn:not, and extends the semantics of effective boolean values to objects, arrays and nulls. The following explanations, provided as an informal summary for convenience, are non-normative.
Figure 35. OrExpr
Figure 36. AndExpr
Figure 37. NotExpr
JSONiq logics support is based on two-valued logics: just true and false.
Non-boolean operands get automatically converted to either true or false, or an error is raised. The boolean() function performs a manual conversion.
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.
An operand singleton sequence whose first item is an object or array is converted to true.
Other operand sequences cannot be converted and an error is raised.
JSONiq supports the most famous three boolean operations: conjunction, disjunction and negation. Negation has the highest precedence, then conjunction, then disjunction. Parentheses can override.
Example 58. Logics with booleans
true and ( true or not true )
Result (run with Zorba): true
Example 59. Logics with comparing operands
1 + 1 eq 2 or 1 + 1 eq 3
Result (run with Zorba): true
Example 60. Conversion of the empty sequence to false
boolean(())
Result (run with Zorba): false
Example 61. Conversion of null to false
boolean(null)
Result (run with Zorba): false
Example 62. Conversion of a string to true
boolean("foo"), boolean("")
Result (run with Zorba): true false
Example 63. Conversion of a number to false
0 and true, not (not 1e42)
Result (run with Zorba): false true
Example 64. Conversion of an object to a boolean (not implemented in Zorba at this point)
{ "foo" : "bar" } or false
Result (run with Zorba): true
If the input sequence has more than one item, and the first item is not an object or array, an error is raised.
Example 65. Error upon conversion of a sequence of more than one item, not beginning with a JSON item, to a boolean
( 1, 2, 3 ) or false
Result (run with Zorba): An error was raised: invalid argument type for function fn:boolean(): effective boolean value not defined for sequence of more than one item that starts with "xs:integer"
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 return an error.
Example 66. Non-determinism in presence of errors.
true or (1 div 0)
Result (run with Zorba): true
JSONiq follows the W3C standard for quantified expressions. The following explanations, provided as an informal summary for convenience, are non-normative.
Figure 38. QuantifiedExpr
It is possible to perform a conjunction or a disjunction on a predicate for each item in a sequence.
Example 67. Universal quantifier
every $i in 1 to 10 satisfies $i gt 0
Result (run with Zorba): true
Example 68. Existential quantifier on several variables
some $i in -5 to 5, $j in 1 to 10 satisfies $i eq $j
Result (run with Zorba): true
Variables can be annotated with a type. If no type is specified, item* is assumed. If the type does not match, an error is raised.
Example 69. Existential quantifier with type checking
some $i as integer in -5 to 5, $j as integer in 1 to 10 satisfies $i eq $j
Result (run with Zorba): true