Defined

&df{x}

This atom may be used in the body to reason about whether a given variable x is defined or not. This is useful for instance to provide defaults or detect errors if a certain integer variable does not have a value but should.

Example

For instance, take program

price(frame,15).

default_price(20).

selected(frame).
{ selected(bag) }.

&sus{V} = price(P)    :- selected(P), price(P,V).
&sus{price(P) : selected(P)} =: calc_price(total).

&sus{price(total)} = calc_price(total) :- &df{calc_price(total)}.
&sus{price(total)} = D                 :- not &df{calc_price(total)},
                                        default_price(D).

#show selected/1.
&show{price/1}.

first tries to calculate the price, and if this calculation has a defined outcome, it is assigned to the total price. If not, a default price is used.

Calling

flingo examples/config_default_in.lp 0

yields the intended two answer sets

Answer: 1
selected(frame) selected(bag) val(price(frame),15) val(price(total),20)
Answer: 2
selected(frame) val(price(frame),15) val(price(total),15)

where Answer 1 makes use of the default because the price of the bag is missing and therefore the total price may not be calculated.