Prolog built-ins and flexible probabilities¶

ProbLog2 supports several other features like flexible probabilities and Prolog built-ins. Let’s illustrate this with the following example.

Suppose we are packing our bags to go on a trip. We have a set of items, each having a particular weight, and we pack each item with probability inversely proportional to its weight. We want to compute the probability that we will have excess baggage, i.e., that the total weight of our baggage will exceed a given limit. We can model this with the following ProbLog program.

Note that this program uses several Prolog built-ins such as support for lists and arithmetic. The program also uses another feature of ProbLog2, namely support for (intensional) probabilistic facts with a `flexible’ probability. This means that the probability is not prespecified but is an arithmetic expression that needs to be computed. In the program, this is used in the intensional probabilistic fact “P::pack(Item) :- …”, which says that the probability of packing an item is inversely proportional to its weight. Such a flexible probability can be used in ProbLog2 under the restriction that the arithmetic expression can be evaluated at call-time (i.e., by the time the probabilistic fact is reached by SLD resolution to prove the queries and evidence).

weight(skis,6). weight(boots,4). weight(helmet,3). weight(gloves,2). % intensional probabilistic fact with flexible probability: P::pack(Item) :- weight(Item,Weight), P is 1.0/Weight. excess(Limit) :- excess([skis,boots,helmet,gloves],Limit). % all possible items excess([],Limit) :- Limit<0. excess([I|R],Limit) :- pack(I), weight(I,W), L is Limit-W, excess(R,L). excess([I|R],Limit) :- \+pack(I), excess(R,Limit). query(excess(8)).

When pressing ‘Evaluate’, ProbLog2 calculates the probability of having excess baggage given a weight limit of 8. We obtain P(excess(8)) = 0.118055.

Supported prolog built-ins documentation

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