empty-list.taq

$ java -jar taq.jar empty-list

Running query empty_list in global scope 
empty_list(value=NaN, backward=-1, inc=NaN, dec=NaN, index=-1, isFact=false)

Description

empty-list.taq demonstrates that a cursor deals gracefully when assigned to an empty list. The cursor declaration is normal, but the “high_cities” axiom list to which it is assigned is not populated before a sequence of operations is performed

cursor city(high_cities)

Note that NaN is a literal value which allows a query to fail gracefully when it is encountered in arithmetic operations.

• Value at current cursor location: city = NaN
• List index after reversing direction: -city = -1
• Value post increment: city++ = NaN
• Value post decrement: city-- = NaN
• Current list index: city.index() = -1
• Fact status is false: fact city = false