The revenge of Prolog

I picked up a copy of Clocksin and Mellish, the "Programming in Prolog" text from 1981. (Actually, it's the second edition from 1984.)

This tome gave me a great deal of difficulty back in the late 1980s. For some reason, I just could not "get" the idea of the Prolog language.

Reading it now, however, is another story. Now I "get" the idea of Prolog. A few years of experience can have a large effect on one's comprehension of "new" technology.

What strikes me about Prolog is that it is not really a programming language. At least, not in the procedural sense. (And it isn't. Clocksin and Mellish state this up front, in the preface to the first edition.)

If Prolog isn't a programming language, then what is it? As I see it, Prolog is a language for interrogating databases. Not SQL databases with tables and rows and columns, but databases that hold bits of information that Clocksin and Mellish call "facts".

For example, one can define a set of facts:

male(albert).
male(edward).
female(alice).
female(victoria).
parents(edward, victoria, albert).
parents(alice, victoria, albert).

Then define a relationship:

sister_of(X,Y) :- female(X), parents(X, M, F), parents(Y, M, F).

And then one can run a query:

sister_of(alice, edward).

Prolog will answer "yes", working out that Alice is the sister of Edward.

Today, we would call this database of facts a NoSQL database. And that is what the Prolog database is. It is a NoSQL database, that is, a collection of data that does not conform (necessarily) to the strict schema of a relational database. The Prolog database is less capable than modern NoSQL databases: it is limited to text items, numeric items, and aggregations of items. Modern NoSQL databases can hold these and more: pictures, audio files, and all sorts of things.

Finding an early version of the NoSQL database concept in the Prolog language is a pleasant surprise. For me, it validates the notion of NoSQL databases. And it validates the idea of Prolog. Great minds think alike, and similar solutions to problems, separated by time, confirm the value of the solution.