Picat Pattern Matching Rules Questions and Answers

Question 1

A developer defines a predicate to verify if the two elements of a pair structure are identical. Which rule head correctly implements this check using only pattern matching?

Accepted Answer

check_equal($p(X, X)) => true.

Answer

check_equal($p(X, Y)) => X == Y.

Answer

check_equal($p(X, Y)) => X = Y.

Answer

check_equal($p(X, Y), X == Y).

Question 2

What is the primary purpose of the `@` operator (as-pattern) in a Picat rule head?

Accepted Answer

To create an alias for a complex pattern, binding the entire structure to a single variable while also deconstructing it.

Answer

To perform a type check on the matched term.

Answer

To indicate a pointer or reference to a memory location.

Answer

To suppress unification and force a strict structural match.

Question 3

Consider the following Picat code. What is the output of the query `go`?

```picat
go => classify(item(10, "food"), C), println(C).

classify(item(Val, _), high) => Val > 20.
classify(item(Val, _), medium) => Val > 5.
classify(item(_, _), low).
```

Accepted Answer

medium

Answer

high

Answer

low

Answer

An error occurs.

Question 4

Which of the following is an INVALID rule head for a Picat predicate?

Accepted Answer

sum_list([H|T], H + sum_list(T)) => ...

Answer

process(ID, $record(Name, Age@val(V))) => ...

Answer

find_pair([X, X | _]) => ...

Answer

handle_default(Data) => ...

Question 5

A programmer wants to handle lists, with a special case for lists starting with the atom 'a'. What is the result of the query `handle_list([a,b,c], Res)` with the following predicate definitions?

```picat
handle_list(L, general) => length(L) > 0.
handle_list([a|_], specific_a).
handle_list([], empty).
```

Accepted Answer

general

Answer

specific_a

Answer

An error due to ambiguity.

Answer

The query fails.

Question 6

In Picat, what is the outcome when a predicate is called with arguments that do not match the pattern of any of its defined rules?

Accepted Answer

The call simply fails, and execution backtracks.

Answer

The program halts with an instantiation error.

Answer

The call implicitly returns a `fail` atom.

Answer

A syntax error is reported at compile time.