Procedural Meaning

The procedural meaning specifies how Prolog answers questions.
Input: a program and a goal list.
Output: a success/failure indicator and an instantiation of variables.
The meaning of the two output results is as follows:
The success/failure indicator is ‘yes’ if the goals are satisfiable and ‘no’ otherwise.
An instantiation of variables is only produced in the case of a successful termination; in the case of failure there is no instantiation.

Declerative Meaning

Declarative reading of the clause “P:-Q,R” are:
P is true if Q and R are true.
From Q and R follows P.
Procedural readings of this clause are:
To solve the problem P, first solve the subproblem Q and then R.
To satisfy P, satisfy Q and then R.


Note that ‘,’ character indicates AND operator.
Note that ‘;’ character indicates OR operator.
AND operator has a higher priority then the OR operator.
“P :- Q, R; S, T, U.” and “P :- (Q, R); (S, T, U).” clauses gives the same results.
Same clauses can be written as:
P :- Q, R.
P :- S, T, U.