Picat Pattern Matching Rules 4 — Questions and Answers
Question 1: In Picat, what is the purpose of writing multiple clauses for the same predicate with different patterns?
- To dispatch behavior based on the structure or value of arguments (Correct answer)
- To define overloaded arithmetic operators
- To create parallel execution branches
- To declare optional parameters
Correct answer: To dispatch behavior based on the structure or value of arguments
Multiple clauses with different patterns implement structural dispatch — Picat picks the clause whose pattern unifies with the actual arguments.
Question 2: What does `[H|T] = [a, b, c]` produce in Picat?
- H = a, T = [b, c] (Correct answer)
- H = [a], T = [b, c]
- H = a, T = b
- A type error because `=` is not for lists
Correct answer: H = a, T = [b, c]
The unification `[H|T] = [a,b,c]` binds H to the head `a` and T to the tail `[b,c]`.
Question 3: Which of the following is a valid Picat rule that only matches when argument N is positive?
- positive(N), N > 0 => write(N). (Correct answer)
- positive(N) => N > 0, write(N).
- positive(N) :- N > 0.
- positive(N) when N > 0 => write(N).
Correct answer: positive(N), N > 0 => write(N).
In Picat rule syntax, the guard `N > 0` is placed after the head pattern and before the `=>` neck, separated by a comma.
Question 4: In Picat, what happens when you use `=` to match a pattern inside a rule body rather than in the head?
- It performs unification at that point in execution, failing if patterns don't match (Correct answer)
- It assigns a new value, overwriting the old binding
- It is equivalent to `==` for comparison only
- It always succeeds and returns true
Correct answer: It performs unification at that point in execution, failing if patterns don't match
The `=` operator in Picat's body is unification — it tries to make both sides identical and throws an error if it fails.
Question 5: If a Picat function clause has the head `f(0)` and a body, for which calls will this clause be tried?
- Only calls where the argument unifies with 0 (Correct answer)
- All calls to f regardless of argument
- Only calls where the argument is a positive integer
- Only calls at the top level
Correct answer: Only calls where the argument unifies with 0
The literal `0` in the head pattern means the clause is only attempted when the first argument can unify with (equals) 0.
Question 6: In Picat, what does `_` (underscore) guarantee about variable binding?
- It matches any value and is never bound, even if it appears multiple times (Correct answer)
- It binds to the first value it sees and reuses that binding
- It must match an uninstantiated variable
- It only matches atoms
Correct answer: It matches any value and is never bound, even if it appears multiple times
Each occurrence of `_` is a fresh anonymous variable that matches anything without creating a binding or linking occurrences.
Question 7: Which pattern matches a non-empty list in Picat?
- [_|_] (Correct answer)
- []
- [_]
- list(_)
Correct answer: [_|_]
`[_|_]` matches any list with at least one element, binding both head and tail to anonymous variables.
Picat Pattern Matching Rules 4
In Picat, what is the purpose of writing multiple clauses for the same predicate with different patterns?