Datalog vs CLIPS vs Prolog

Question

As many programmers I studied Prolog in university, but only very little. I understand that Prolog and Datalog are closely related, but Datalog is simpler? Also, I believe tha

Answer 1

The difference between CLIPS and Prolog/Datalog is that CLIPS is a "production rule system" that operates by forward chaining: given a set of facts and rules, it will try to make every possible derivation of new facts and store those in memory. A query is then answered by checking whether it matches something in the fact store. So, in CLIPS, if you have (pseudo-syntax):

parent(X,Y) => child(Y,X)
parent(john,mary)

it will immediately derive child(mary,john) and remember that fact. This can be very fast, but puts restrictions on the possible ruleset and takes up memory.

Prolog and Datalog operate by backward chaining, meaning that a query (predicate call) is answered by trying to prove the query, i.e. running the Prolog/Datalog program. Prolog is a Turing complete programming language, so any algorithm can be implemented in it.

Datalog is a non-Turing complete subset of Prolog that does not allow, e.g., negation. Its main advantage is that every Datalog program terminates (no infinite loops). This makes it useful for so-called "deductive databases," i.e. databases with rules in addition to facts.