Pattern Matching Rules Flashcards

In Picat, what does the `?=>` operator signify in a rule head compared to `=>`?

Answer: `?=>` is non-deterministic (allows backtracking); `=>` is deterministic (commits on match)

Rules using `?=>` are non-deterministic and allow the system to backtrack and try other clauses, while `=>` commits to the first matching clause.

Which Picat pattern correctly matches a list with exactly two elements?

Answer: [X, Y]

`[X, Y]` matches a list of exactly two elements, binding X to the first and Y to the second.

In a Picat rule, a guard expression appears:

Answer: After the rule head, preceded by a comma, before the neck operator

A guard is a condition placed after the pattern head and before the `=>` or `?=>` neck, separated by a comma.

What happens when no pattern in a Picat function definition matches the given arguments?

Answer: A runtime exception is thrown

If no clause pattern matches the arguments, Picat throws an existence error or no-clause exception at runtime.

Which pattern in Picat uses an anonymous variable to ignore a list element?

Answer: [_, X | _]

The underscore `_` is Picat's anonymous variable that matches any value without binding it.

Consider `foo([H|T]) => write(H).` — what does H bind to when called as `foo([1,2,3])`?

Answer: 1

In the head pattern `[H|T]`, H binds to the first element (head) of the list, which is 1.

In Picat pattern matching, what does matching against an integer literal in the head accomplish?

Answer: It restricts the rule to fire only when the argument equals that exact integer

A literal in a pattern head acts as an equality constraint, so the rule only fires if the argument unifies with that exact value.