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Programming Languages

Prolog Introduction

Readings

Logic programming

Prolog is a logic programming language, i.e., its programs consist of of a set of facts and logic rules. Computation happens by deriving new facts from these rules. Since facts are a set, the order in which they are given is irrelevant.

Prolog was developed in France in the early 1970s by Alain Colmeraeur and Philippe Roussel. It was first intended for natural language processing, but throughout the years has been applied in many other fields, as it is well-suited for any application amenable to rule-based logical queries.

Terms

Prolog statements are written using terms. A term is either:

  • an atom: is an identifier, starting with lower case, without a predetermined meaning. E.g.: atom, x, y.
  • a number: a float or integer string. E.g.: 123, -5.5.
  • a variable: an identifier, starting with upper case, or an underscore. They are placeholders for arbitrary terms. E.g.: X, Var, _
  • a composite term: an atom followed by a sequence of terms between parentheses. E.g.: x(Y, Z), x(y, Z).

Facts

A fact is written as a term followed by a period: parent(kim, holly).

Writing a fact establishes that something holds, i.e., is true. In the above kim and holly are atoms and parent is a predicate, a function that is true when applied to kim and holly.

We can start writing a Prolog program by writing a series of facts:

parent(kim, holly).
parent(margaret, kim).
parent(margaret, kent).
parent(esther, margaret).
parent(herbert, margaret).
parent(herbert, jean).

A file with the above program, say parent.pl, can be loaded into swipl, a Prolog interpreter, with

?- [parent].

which will evaluate the terms within it, letting the Prolog interpreter know that these facts hold.

Queries

One can then interact with the interpreter with queries:

?- parent(kim, holly).
?- parent(kim, X).
?- parent(X, Y).
?- parent(Y, esther).

Note that in querying terms with variables the interpreter will instantiate free variables with terms so that the ground predicate (one without variables) holds. Pressing space or typing ; (the symbol for disjunction) yields another instantiation, if any.

?- parent(X,margaret).
X = esther ;
X = herbert.

Queries can be written as sequences of terms, conjunctively or disjunctively:

?- parent(margaret,X), parent(X, holly).
?- parent(margaret,X); parent(X, holly).

We can start getting creative: how to query whether esther has a great-grandchild?

?- parent(esther, C), parent(C, GC), parent(GC, GGC).

For this query to hold, all predicates must hold. For each to hold, the variables must be assigned terms so that the respective ground predicate holds. There is only one way:

C = margaret,
GC = kim,
GGC = holly

Rules

A rule specifies how new facts can be derived once some conditions are met.

greatGrandparent(GGP, GGC) :- parent(GGP, GP), parent(GP, P), parent(P, GGC).

The term to the left of :- is true if the term to the right is true. Rules can be seen as definitions for predicates: the greatGrandparent predicate holds for terms GGP and GGC when GGP has a child who has a child who is GGC.

Rules can be composed:

grandparent(GP, GC) :- parent(GP, P), parent(P, GC).
greatGrandparent(GGP, GGC) :- grandparent(GGP, P), parent(P, GGC).

Note that the order is irrelevant. The above is equivalent to

greatGrandparent(GGP, GGC) :- grandparent(GGP, P), parent(P, GGC).
grandparent(GP, GC) :- parent(GP, P), parent(P, GC).

Rules can be defined recursively:

ancestor(X, Y) :- parent(X, Y).
ancestor(X, Y) :- parent(Z, Y), ancestor(X, Z).

If we were to define a siblings predicate we could do:

sibling(X,Y) :- parent(P,X), parent(P, Y).

However note that X and Y can be instantiated to the same term, which does not correctly model our intuition of when the sibling predicate should hold. We can change it to:

sibling(X,Y) :- parent(P,X), parent(P, Y), not(X=Y).

Lists

Prolog has built-in list terms. One write lists as terms separated by commas within square brackets.

?- X = [1, haniel, true].
?- [1,2|X] = [1,2,3,4,5].

Appending

We can define a predicate that appends two lists:

append([], L, L).
append([Head|TailA], B, [Head|TailC]) :- append(TailA, B, TailC).

The first rule (a fact) defines the base case, in which appending the empty list to another list L is L itself. The second rule, from a non-empty list [Head|TailA] and another list B, builds the list [Head|TailC] whose head is the head of the first list and the tail is the result of appending the tail of the first list with the second list.

?- append([1,2,3], [4,5,6], X).
?- append(X, Y, [1,2,3]).

Membership

We can define a member predicate:

member(X, [X|_]).
member(X, [_|Y]) :- member(X, Y).

A term is part of a list if it’s equal to its head (first rule), otherwise it’s necessary to search in its tail (second rule).

?- member(2, [1,2,3]).
?- member(2, [X,2,3]).
?- member(2, [1,X,3]).
?- member(4, [1,2,3]).
?- member(X, [1,2,3]).
?- member(1, X).

Selection

We can define a select predicate such that select(E, L1, L2) is true if and only if L2 is the same as L1, minus an element E:

select(X, [X|L], L).
select(X, [H|L], [H|LR]) :- select(X, L, LR).

If the element is the head of the list, the result is its tail (first rule). Otherwise the result is the selection of the elemen in the tail of the list (second rule). If the list is empty the predicate is false.

?- select(2, [1,2,3], Z).
?- select(2, [1,3], Z).
?- select(X, [1,2,3], [2,3]).
?- select(X, [1,2,3], Y).

Reverse

reverse([], []).
reverse([H|T], X) :- reverse(T, XAux), append(XAux, [H], X).

?- reverse([1, 2, 3], L).

What is the expected result of ?- reverse(L, [1, 2, 3]).? The second argument of the reverse predicate is the reverse of the first argument. Therefore for this query to hold L must be the reverse of [1, 2, 3].

Permutation

We can define a predicate perm such that perm(T, L) is true iff L is a permutation of the list T:

perm([], []).
perm(T, [H|LL]) :- select(H, T, L1), perm(L1, LL).

Puzzle

How to use Prolog to solve the Wolf, goat and cabbage problem? A detailed solution is available here.