Picat Pattern Matching Rules 3

Question 1

What is the result of matching the pattern `f(X, X)` against the term `f(3, 4)` in Picat?

Accepted Answer

Match fails because X cannot unify with both 3 and 4

Answer

When the same variable appears twice in a pattern, both positions must unify to the same value; since 3 ≠ 4, the match fails.

Question 2

In Picat, which rule type is searched in order and allows multiple clauses to be tried on backtracking?

Accepted Answer

Non-deterministic rules using `?=>`

Answer

Non-deterministic rules (`?=>`) allow Picat to backtrack and try subsequent clauses when the current one fails.

Question 3

How do you write an as-pattern in Picat to bind the whole list while also matching its head?

Accepted Answer

L@[H|_]

Answer

The `@` syntax (e.g., `L@[H|_]`) creates an as-pattern, binding L to the full term while destructuring it simultaneously.

Question 4

In Picat, if two clauses have the same head pattern and neither has a guard, which one executes?

Accepted Answer

The first textually defined clause

Answer

Picat searches clauses top-to-bottom; the first matching clause wins (and with `=>` commits, preventing backtracking to the second).

Question 5

What does the pattern `{X, Y, Z}` match in Picat?

Accepted Answer

A tuple with exactly three elements

Answer

Curly braces in Picat denote tuples, so `{X, Y, Z}` matches a three-element tuple and binds each position.

Picat Practice Test