Posted on 28 Jan 2014
Over the past couple days, I’ve been banging my head against the wall, trying to understand the logic programming language Prolog (or more specifically, SWI-Prolog) for our CS162 Programming Languages class.
What is Prolog?
Prolog is a declarative programming language. It operates by performing queries against given facts and relations, somewhat similar to database query languages like SQL. An example of Prolog:
man(adam). man(peter). woman(alice). woman(mary). woman(sara). parent(adam, peter). parent(adam, mary). parent(adam, sara). parent(alice, peter). parent(alice, mary). parent(alice, sara). father(F, C) :- man(F), parent(F, C). mother(M, C) :- woman(M), parent(M, C). siblings(A, B) :- parent(P, A), parent(P, B), A \= B.
This example illustrates a family of 5; Adam and Alice have three children, named Peter, Mary, and Sara.
In Prolog, identifiers beginning with lowercase letters are known as atoms or
adam is an atom.
Identifiers beginning with capital letters are variables.
C are a
few of the variables in this example. Note that variables can be more than one
Functions in the language are known as predicates; they take one or more parameters and return a boolean value.
In the above example,
siblings are obviously
predicates. But notice
parent. These are also
man(adam). signals that when the predicate
man is used with atom
adam, it will always return true.
What’s interesting is that predicates can be called with an uninitialized variable. In this case, Prolog conducts a search for all possible values for that variable that cause the predicate to return true. For example, the query:
?- siblings(peter, Siblings).
To programmers more familiar with the imperative style, Prolog presents a vastly different perspective.
Some resources I found useful for learning Prolog:
- Learn Prolog Now! – Great tutorial for getting started.
- SWI-Prolog Manual – SWI-Prolog manual pages provide a quick reference for looking up functions and parameters.
- StackOverflow – As with anything else programming related, StackOverflow is here to help.