Tracing a Prolog program

• When we make the query :
  
  nice.
• nice becomes the goal to be satisfied.
• Next, warm becomes the goal to be satisfied, as warm is the first clause Prolog finds.
• Prolog again searches the program to find a clause whose head matches the desired goal, warm
• Two goals are found in the body of the rule to be satisfied : temperature (T) and T > 20. Prolog attempts to satisfy them left to right. When fact temperature(15) is encountered, Prolog tries to satisfy the goal 15 > 20. This is not possible, so goal fails and Prolog automatically backtracks to the last goal that was matched, i.e. temperature (T), and tries to perform this match in another way. While backtracking (T) gets uninstantiated.
• To satisfy the goal temperature (T), Prolog does not start search from the beginning but continues from the clause where this goal was matched last. It starts with the fact temperature (15), and searches for another clause with head of the same name (temperature). = it fails.
• Prolog again automatically backtracks to the last goal that was successfully matched i.e. to the goal warm. Now it tries to find an alternative clause whose head would match this goal. Search continues from the point where the last match occured, and not from the beginning. Since another clause for warm cannot be found, Prolog backtracks to nice
• A new clause is now found !!
• Execution is now performed as above, with the goal to be satisfied now being sunny and not_windy. The goal sunny is satisfied by matching it with the fact sunny.
• Similarly the goal not_windy is matched with the relevant clause
• Now, the two goals to be satisfied are wind_speed(S) and S < 5, which are compared with the fact wind_speed(3) and the goal 3 < 5, which is the final goal

The execution of a Prolog program is therefore non-deterministic because of multiple execution paths. Of course, if all procedures in a program consisted of only one clause, then only one execution path would exist

• The closed world assumption
• Prolog program as a set of axioms (as facts and rules) from which we can prove theorems (get answer to questions) using logical deductive reasoning
• We state a theorem and Prolog tries to prove it (derive it) from the given axioms (program). Hence, the concept of Prolog as a theorem prover.

Useful to draw search trees to understand the operation of Prolog in backtracking - Graphical representations always prefered.