Prolog: Completely, 100% useless right up until it’s exactly the right tool for a very hard, very specific job. (source)

Prolog is a programming language that allows us to “program” with declarative, symbolic knowledge. It is a Turing-complete language, and thus supports loops, variables, etc. but Prolog programs consist primarily of “facts” and “rules” that state the relations between facts. The rules are activated by “queries”. By using variables in the queries, we can find out the values of the variables that make the queries true (according to the facts and rules involved), and thus we get a kind of computation.

Install SWI Prolog or run it on londo with the command swipl.

Logic programming paradigm

Because Prolog is a “Turing-complete” language, we can write any program in Prolog. Of course, a program that is easy to write in Java or C++ will, in all likelihood, be very difficult to write in Prolog, although it is possible. Prolog programs often feel like they’re programs written in “reverse,” since they are written declaratively rather than procedurally.

  • Prolog programs have two parts: a database (of facts and rules), and an interactive “query” tool; the database must be saved in a text file, usually with extension .pl.

  • Prolog databases are “consulted” (loaded), and then the query tool is used to “make queries” (ask questions) about the database.

  • How queries are answered is mostly beyond the control of the programmer; Prolog uses a depth-first search to figure out how to answer queries; this is (mostly) non-negotiable.

  • Programs written in Prolog are “executed” by performing queries. By using variables in queries, we get computation.

Note, Prolog rules can also include input/output statements, so just evaluating a rule with a query can result in computation as well.

Simple example

Let’s make a database by creating some facts:

parent(tom, liz).
parent(bob, ann).
parent(bob, pat).
parent(pat, jim).


We type that into a file, maybe named family.pl, and “consult” the file in the query tool, like so:

$ nano family.pl # type the file, then quit nano
$ pl             # start prolog
?- [family].     # "consult" (load) the file

Now what? Well, we can ask questions:

?- female(pam).

Ok, it’s true that pam is a female.

?- parent(pat, ann).

Right, pat is not ann’s parent.

Now let’s use variables. Variables always start with capital letters. Objects (like pam) must not.

?- parent(X, ann).
X = bob.

We see that Prolog has found an assignment for X that makes the statement true. We call parent a predicate (a 2-ary predicate because it accepts two inputs), and we always write queries by first stating a predicate.

Let’s ask, “Who are all the parents?”

?- parent(P, C).
P = tom,
C = liz ;
P = bob,
C = ann ;
P = bob,
C = pat ;
P = pat,
C = jim.

We type ; to get the next answer. Semicolon means “or” while comma means “and.”

If we don’t actually care who the children are, we can replace C with _ like so:

?- parent(P, _).
P = tom ;
P = bob ;
P = bob ;
P = pat.

We get bob twice because he’s the parent of two people.

Now let’s ask, “Who are the female parents?”

?- parent(P, _), female(P).
P = pat.

The comma means “and.” So the query requires that P is a parent and a female.

Defining complex predicates (rules)

The predicates shown above (parent, male, and female) are always true. They are called “facts.”

A “rule,” instead, is true only when some conditions are met. These conditions look like the last query above: they typically require several predicates to be true. The last query above is basically what defines a “mother.” Let’s make a rule (in our database file, family.pl):

mother(X) :- parent(X, _), female(X).

“Consult” the database again, and run these queries:

?- mother(pat).

?- mother(jim).

?- mother(X).
X = pat.

Now for fathers. In the database, we add:

father(X) :- parent(X, _), male(X).

And here is a query:

?- father(X).
X = tom ;
X = bob ;
X = bob ;

Why the false at the end? In the context of an interactive query, ; means “throw away the answer you just gave, and try again.” Prolog finds a parent X (probably pat), but then that X does not satisfy the second part of the father rule, which is written male(X), so the answer is false for that X.

Notice that pat is the parent of jim and bob is the parent of pat. That makes bob a grandparent. Let’s write a query to that effect:

?- parent(X, Y), parent(Y, _).
X = bob,
Y = pat.

See? bob is a grandparent. Let’s make a rule:

grandparent(X) :- parent(X, Y), parent(Y, _).

And here are two queries:

?- grandparent(bob).

?- grandparent(jim).

Let’s try defining the sister relationship:

sister(X, Y) :- parent(Z, X), parent(Z, Y), female(X), female(Y).

And some queries:

?- sister(ann, pat).

?- sister(bob, pat).

?- sister(X, Y).
X = Y, Y = liz ;
X = Y, Y = ann ;
X = ann,
Y = pat ;
X = pat,
Y = ann ;
X = Y, Y = pat ;

What does X = Y, Y = liz mean? It means liz is, apparently, her own sister:

?- sister(liz, liz).

Let’s prevent that by adding another clause to the sister predicate (\= means “not equal”):

sister(X, Y) :- parent(Z, X), parent(Z, Y), female(X), female(Y), X \= Y.

Now, we get the correct behavior:

?- sister(liz, liz).

?- sister(X, Y).
X = ann,
Y = pat ;
X = pat,
Y = ann ;

We can simulate an “or” by writing multiple definitions for the same rule:

engineer(X) :- mechanical_engineer(X).

engineer(X) :- chemical_engineer(X).

engineer(X) :- electrical_engineer(X).

% etc. etc.

In each rule, we use commas to represent “and,” but (generally) do not write “or” inside a rule. When required, which is rare, we can use semicolons.

Negation as failure

Negation is a little bit funny in Prolog. That’s because negation is a little bit funny in life. How does anyone prove a negative statement? “I never read that book” — how can that be proved?

Let’s say we want a predicate like “this person is not a father.” Well, we know how to figure out who is a father: we have the father rule. How can we define a “is not a father” rule?

The first thing to note is Prolog does not allow us to use “not” at the head (left side) of a rule. We cannot write “not father(jim).”

Instead, to prove a negative in Prolog, we use a little trick: we assume everything stated in the database is everything that exists. This is the closed-world assumption. Any fact not stated in the database is assumed to be false. So, we can “prove” that jim is not a father by failing to prove that jim is a father (with our father rule).

not_a_father(X) :- \+father(X).

Some queries:

?- not_a_father(bob).

?- not_a_father(jim).

The \+ is the “negation operator.” The idea is that \+father(X) is true whenever father(X) fails to be proved (recall that Prolog may ultimately search every possibility in order to prove something; thus, negation can be computationally expensive).

We cannot use \+ on the left-side of a rule or written as a fact because it makes no sense to say, “this will fail to be proved always.” Only positive statements may be on the left of a rule or may stated as a fact.

This connection between “failing to prove” and “negation” is called “negation as failure.” Negation as failure means, “proving the negation of something is equivalent to failing to prove that something” and depends on a closed-world assumption.

negation-as-failure Also negation-by-failure n. Logic programming 1 The reasonable assumption that $P$ is false if one has failed to prove that $P$ is true. 2 The unreasonable assumption that $P$ is false if others have failed to prove that $P$ is true. 3 Drugs The ineffectiveness of the “Just Say No!” campaign. — The computer contradictionary

Lists in Prolog

Besides atoms (ann, pat, bob, etc.) we can create lists. Let’s make a simple predicate that says whether or not something is at the head (front) of a list:

head(X, [X|_]).

The code [X|_] means there is a list that starts with X and the rest (after the |) we don’t care about (hence the _). Here are some uses of this predicate:

?- head(a, [a, b, c]).

?- head(X, [a, b, c]).
X = a.

?- head(a, X).
X = [a|_G228].

The _G228 is a variable that Prolog created to represent the rest of the list; it has made no commitment about what _G228 is, hence it can only describe it as variable _G228.

Predicates can be recursive, which is exactly what is needed for the membership predicate:

% X is a member of a list if the list only has the element X in it
member(X, [X]).

% X is a member of a list if the list starts with X
% (technically, the above statement is not needed; this
% statement suffices)
member(X, [X|_]).

% Finally, X is a member of a list if it's a member of
% the tail of the list
member(X, [_|Tail]) :- member(X, Tail).

Example usage (we type . after some responses because we don’t want other possible answers):

?- member(a, [a]).
true .

?- member(a, [a, b, c]).
true .

?- member(X, [a, b, c]).
X = a ;
X = b ;
X = c .

?- member(a, X).
X = [a|_G231] ;
X = [_G230, a|_G234] ;
X = [_G230, _G233, a|_G237] ;
X = [_G230, _G233, _G236, a|_G240] ;
X = [_G230, _G233, _G236, _G239, a|_G243] .

In that last case, Prolog is generating lists that have a as a member somewhere. Pretty weird, huh? It’s finding some way to make the predicate true.

Prolog for databases

Now we’ll investigate using Prolog for representing and answering queries about data. Let’s build a database of people and families. We want to say that certain people are born on certain dates, have jobs with particular incomes, are married, have children, etc.

We’ll start with a predicate, family, that says a family is made up of an ID (a number), two parents, and zero-or-more children (a list). Each person has a first name and last name, birth date, and is either unemployed or has a job with a particular salary.

       person(tom, fox, born(7, may, 1960), works(cnn, 152000)),
       person(ann, fox, born(19, april, 1961), works(nyu, 65000)),
       % here are the children...
       [person(pat, fox, born(5, october, 1983), unemployed),
        person(jim, fox, born(1, june, 1986), unemployed),
        person(amy, fox, born(17, december, 1990), unemployed)]).

       person(susan, rothchild, born(13, september, 1972), works(stetson, 75000)),
       person(jess, rothchild, born(20, july, 1975), works(nationwide, 123500)),
       % here are the children...
       [person(ace, rothchild, born(2, january, 2010), unemployed)]).

There are two families. Note that person is not a predicate, it’s just a way of writing data. We have no way of asking person(X, Y, ...) because it’s not a predicate. To access person data, we have to use the family predicate. Here are some examples:

?- family(38463, X, Y, Z).
X = person(susan, rothchild, born(13, september, 1972), works(stetson, 75000)),
Y = person(jess, rothchild, born(20, july, 1975), works(nationwide, 123500)),
Z = [person(ace, rothchild, born(2, january, 2010), unemployed)].

?- family(38463, person(FirstName, LastName, _, _), _, _).
FirstName = susan,
LastName = rothchild.

?- family(ID, person(susan, rothchild, _, _), _, _).
ID = 38463.

Using this family predicate, we just put a variable in a location that we want to retrieve information about (such as family ID or a first name / last name of a person), and put _ for all the slots we don’t care about.

Now let’s make it a little more interesting. Here is a married predicate that says if two people are married:

married(FirstName1, LastName1, FirstName2, LastName2) :-
    family(_, person(FirstName1, LastName1, _, _),
           person(FirstName2, LastName2, _, _), _).

married(FirstName1, LastName1, FirstName2, LastName2) :-
    family(_, person(FirstName2, LastName2, _, _),
           person(FirstName1, LastName1, _, _), _).

And its use:

?- married(tom, fox, ann, fox).
true .

?- married(X, Y, ann, fox).
X = tom,
Y = fox .

?- married(X, Y, A, B).
X = tom,
Y = fox,
A = ann,
B = fox ;
X = susan,
Y = rothchild,
A = jess,
B = rothchild .

Here is a predicate for calculating household income. We can calculate (simple math) using the is keyword:

householdIncome(ID, Income) :-
    family(ID, person(_, _, _, works(_, Income1)),
           person(_, _, _, works(_, Income2)), _),
    Income is Income1 + Income2.

And its use:

?- householdIncome(38463, X).
X = 198500.

?- householdIncome(X, 198500).
X = 38463.

?- householdIncome(X, 999).

?- householdIncome(X, Y).
X = 10392,
Y = 217000 ;
X = 38463,
Y = 198500.

To facilitate further queries, we’ll create an exists predicate that we can use to retrieve details about a person. It just pulls person information out of the various places it may appear in a family fact.

exists(Person) :- family(_, Person, _, _).
exists(Person) :- family(_, _, Person, _).
exists(Person) :- family(_, _, _, Children), member(Person, Children).

We can use it like so:

?- exists(X).
X = person(tom, fox, born(7, may, 1960), works(cnn, 152000)) ;
X = person(susan, rothchild, born(13, september, 1972), works(stetson, 75000)) ;
X = person(ann, fox, born(19, april, 1961), works(nyu, 65000)) ;
X = person(jess, rothchild, born(20, july, 1975), works(nationwide, 123500)) ;
X = person(pat, fox, born(5, october, 1983), unemployed) ;
X = person(jim, fox, born(1, june, 1986), unemployed) ;
X = person(amy, fox, born(17, december, 1990), unemployed) ;
X = person(ace, rothchild, born(2, january, 2010), unemployed).

?- exists(person(X, fox, _, _)).
X = tom ;
X = ann ;
X = pat ;
X = jim ;
X = amy .

?- exists(person(FirstName, LastName, born(_, _, 1972), _)).
FirstName = susan,
LastName = rothchild .

?- exists(person(FirstName, LastName, _, unemployed)).
FirstName = pat,
LastName = fox ;
FirstName = jim,
LastName = fox ;
FirstName = amy,
LastName = fox ;
FirstName = ace,
LastName = rothchild.

Let’s get a little more sophisticated. Here is an interesting query:

% Find all people born after 1980 who do not work at Stetson

?- exists(person(FirstName, LastName, born(_, _, Year), Job)),
      Year > 1980, Job \= stetson.

FirstName = pat,
LastName = fox,
Year = 1983,
Job = unemployed ;
FirstName = jim,
LastName = fox,
Year = 1986,
Job = unemployed ;
FirstName = amy,
LastName = fox,
Year = 1990,
Job = unemployed ;
FirstName = ace,
LastName = rothchild,
Year = 2010,
Job = unemployed.

Let’s add a householdSize predicate. It “counts” the number of children and adds 2 (for the parents):

householdSize(ID, Size) :-
    family(ID, _, _, Children),
    length(Children, ChildrenCount),
    Size is 2 + ChildrenCount.


?- householdSize(38463, Size).
Size = 3.

?- householdSize(X, 5).
X = 10392 .

?- householdSize(X, Size), Size > 3.
X = 10392,
Size = 5 .

We can use an interesting built-in predicate, findall, that finds a list of objects that match some “goal.” Here is its use in a query:

% First part of findall is what to collect,
% second part is the "goal" (a predicate),
% third part is the name of the resulting list
?- findall(FirstName, exists(person(FirstName, _, _, _)), Result).
Result = [tom, susan, ann, jess, pat, jim, amy, ace].

Of course, we can count the list:

?- findall(FirstName, exists(person(FirstName, _, _, _)), Result),
       length(Result, N).
Result = [tom, susan, ann, jess, pat, jim, amy, ace],
N = 8.

If we get a list of numbers, we can sum or average them. We’ll need to load a Prolog library, however. Here is some code in a database file:

:- use_module(library(lists)). % load lists library for sumlist predicate

average(List, Avg) :-
    sumlist(List, Sum),
    length(List, N),
    Avg is Sum / N.

Here is a use:

?- average([1, 3, 4.2, 4.5], Avg).
Avg = 3.175.

Ok, now let’s grab household incomes and average them. In query mode:

?- findall(Income, householdIncome(_, Income), Result), average(Result, Avg).
Result = [217000, 198500],
Avg = 207750.

Prolog by other names

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