Prolog unification

When programming in Prolog, we spend a lot of time thinking about how variables and rules “match” or “are assigned.” There are actually two aspects to this. The first, “unification,” regards how terms are matched and variables assigned to make terms match. The second, “resolution,” is described in separate notes. Resolution is only used if rules are involved. You may notice in these notes that no rules are involved since we are only talking about unification.

Prolog ‘Terms’

Prolog has three kinds of terms:

  1. Constants like 42 (numbers) and franklin (atoms, i.e., lower-case words).

  2. Variables like X and Person (words that start with upper-case).

  3. Complex terms like parent(franklin, bo) and baz(X, quux(Y)).

Two terms unify if they can be matched. Two terms can be matched if:

  • they are the same term (obviously), or

  • they contain variables that can be uniformly instantiated so that the two terms without variables are the same. “Uniformly” instantiated means that each distinct variable gets the same value, e.g., all X variables in the term get the same value.

For example, suppose our knowledge base is:

loves(vincent, angela).
loves(franklin, mia).
  • mia and mia unify because they are the same.

  • mia and X unify because X can be given the value mia so that the two terms (without variables) are the same.

  • woman(mia) and woman(X) unify because X can be set to mia which results in identical terms.

  • loves(X, mia) and loves(vincent, X) cannot unify because there is no assignment for X (given our knowledge base) that makes the two terms identical.

  • loves(X, mia) and loves(franklin, X) also cannot unify (can you see why?).

We saw in the Prolog notes that we can “query” the knowledge base and get, say, all the people who love mia. When we query with loves(X, mia). we are asking Prolog to give us all the values for X that unify. These values are, essentially, the people who love mia.

A more formal definition

term1 and term2 unify whenever:

  1. If term1 and term2 are constants, then term1 and term2 unify if and only if they are the same atom, or the same number.

  2. If term1 is a variable and term2 is any type of term, then term1 and term2 unify, and term1 is instantiated to term2. (And vice versa.) (If they are both variables, they’re both instantiated to each other, and we say that they share values.)

  3. If term1 and term2 are complex terms, they unify if and only if:

    a. They have the same functor and arity. The functor is the “function” name (this functor is foo: foo(X, bar)). The arity is the number of arguments for the functor (the arity for foo(X, bar) is 2).

    b. All of their corresponding arguments unify. Recursion!

    c. The variable instantiations are compatible (i.e., the same variable is not given two different unifications/values).

  4. Two terms unify if and only if they unify for one of the above three reasons (there are no reasons left unstated).


We’ll use the = predicate to test if two terms unify. Prolog will answer “Yes” if they do, as well as any sufficient variable assignments to make the unification work.

Do these two terms unify?

?- mia = mia.

Yes, from rule 1.

Do these two terms unify?

?- mia = X.

Yes, from rule 2.

Do these two terms unify?

?- X = Y.

Yes, from rule 2.

Do these two terms unify?

?- k(s(g), Y) = k(X, t(k)).

Yes, from rule 3 and, in the recursion, from rule 2 in two cases (X set to s(g) and Y set to t(k)).

Do these two terms unify?

?- k(s(g), Y) = k(s(g, X), Y).

No, because rule 3 fails in the recursion (in which rule 3 is invoked again, and the arity of s(g) does not match s(g, X)).

Much of these notes were adapted from Learn Prolog Now!, a free online textbook.

CSCI 431 material by Joshua Eckroth is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License. Source code for this website available at GitHub.