In addition to the standard xs:boolean constructor, xs:boolean values can be constructed using the true and false functions, which take no arguments and return the appropriate value. For example, true( ) returns the value true.

Boolean values are more often constructed indirectly, as:

In addition, a function named boolean can be used to explicitly convert a sequence to its effective Boolean value. A sequence that is an xs:boolean value false, a single number 0 or NaN, a single zero-length string, or the empty sequence, evaluates to false. Otherwise, it evaluates to true. Note that it doesn't give the same result as the xs:boolean constructor—for example, xs:boolean("false") is false, but boolean("false") is true. More information on the boolean function can be found in Appendix A.

Values of type xs:string or xs:untypedAtomic can be cast to xs:boolean. The string false (all lowercase), or the string 0 is converted to the value false. The string true (all lowercase) or 1 is converted to true. Any other string value raises an error; other strings that may appear to be equal to 0 or 1, such as 0.0 or 01, are not accepted.

Values of any of the numeric types can also be cast to and from xs:boolean. A numeric value that is equal to 0 or NaN is converted to false; any other numeric value is converted to true.

Likewise, values of type xs:boolean can be cast to xs:string, xs:untypedAtomic, and any of the numeric types. When they are cast to xs:string or xs:untypedAtomic, they are represented as true and false, not their numeric equivalents.