Picat Logic Programming and Unification Questions and Answers

Question 1

In Picat, what is the fundamental difference between pattern matching in rule heads and explicit unification using the '=' operator in the rule body?

Accepted Answer

Pattern matching in rule heads is a one-sided unification that does not bind variables in the call, whereas '=' performs full unification.

Answer

Picat's rule selection mechanism uses pattern matching, which is a form of one-sided unification. It checks if the call arguments are an instance of the head's patterns without binding variables in the call itself. Any required bindings must be performed explicitly in the rule's body using the '=' operator, which performs full, bidirectional unification.

Question 2

What will be the result of the following Picat query?

`go =>
  X = 1,
  Y = X,
  $f(X, 2) = $f(1, Y),
  println(X),
  println(Y).`

Accepted Answer

It will print 1 and then 1.

Answer

The query first binds X to 1. Then Y is unified with X, so Y is also 1. The unification `$f(X, 2) = $f(1, Y)` succeeds because at this point, it is equivalent to `$f(1, 2) = $f(1, 1)`, which successfully unifies Y to be 2, but since X is already bound to 1 and Y is bound to X, Y becomes 1. The unification `$f(1,2) = $f(1,1)` will succeed, and the final values of X and Y will both be 1. The `println` calls will therefore print 1 and 1.

Question 3

Which of the following statements correctly describes unification in Picat?

Accepted Answer

Unification is the process of finding a substitution of variables that makes two terms identical, and it can fail if no such substitution exists.

Answer

Unification is a core concept in logic programming. It is the process by which variables are instantiated to values or other variables to make two terms structurally identical. If the terms cannot be made identical (e.g., unifying `a` with `b`, or `f(X)` with `g(X)`), the unification goal fails.

Question 4

Consider the following Picat code:

`process_data($point(A, B), Result) => Result = A + B.
process_data([H|_], Result) => Result = H.`

What happens when the query `process_data(Data, R)` is called if `Data` is the structure `$point(5, 10)`?

Accepted Answer

The first rule is chosen through pattern matching, and R is unified with 15.

Answer

Picat's engine will try to match the call `process_data($point(5, 10), R)` with the defined rule heads. The structure `$point(5, 10)` matches the pattern `$point(A, B)`. This is a successful one-sided unification where A gets the value 5 and B gets the value 10 for the scope of the rule. The body `Result = A + B` is then executed, unifying `Result` with `5 + 10`, which is 15.

Question 5

Given the goal `[A, B | C] = [x, y, z, w]`, which set of bindings for the variables A, B, and C is correct after successful unification in Picat?

Accepted Answer

A = x, B = y, C = [z, w]

Answer

In Picat (as in Prolog), the `[Head | Tail]` syntax deconstructs a list. The elements before the `|` are unified with the initial elements of the list on the right. The variable after the `|` is unified with the rest of the list. Therefore, `A` unifies with `x`, `B` unifies with `y`, and `C` unifies with the remaining list `[z, w]`.

Picat Practice Test

Picat Practice Test

Picat Logic Programming and Unification Questions and Answers