Functions vs. Predicates Flashcards

What does Picat's `to_string/1` function do that a predicate could not directly accomplish?

Answer: It returns a string value usable inline in expressions like `print(to_string(42) ++ "!")`

As a function, `to_string/1` returns a value immediately composable with operators like `++`.

In Picat, which of the following correctly adds a guard to a predicate clause?

Answer: classify(X), X > 0 => println(positive)

In Picat, a guard is placed between the head and the `=>` neck using a comma: `Head, Guard => Body`.

Picat functions can be higher-order. Which call correctly applies function `F` to value `X` and uses the result?

Answer: Result = call(F, X)

In Picat, `call(F, X)` invokes a function reference `F` with argument `X` and returns the result.

What is the key reason you cannot use a predicate directly inside a list comprehension expression in Picat?

Answer: List comprehensions expect expressions that yield values, which only functions provide

List comprehension expressions like `[f(X) : X in L]` require `f` to return a value, so `f` must be a function.

A Picat module exports both `area/1` (function) and `print_area/1` (predicate). Which import style correctly handles both?

Answer: import geometry

`import ModuleName` imports all exported functions and predicates from the module.

Consider `abs(X) = X, X >= 0 => X` and `abs(X) = -X`. What design pattern does this demonstrate?

Answer: Guarded function clauses with a fallback

The first clause has a guard `X >= 0`; if it fails, the second clause serves as the fallback.

Which statement best describes how Picat handles a predicate that is called but immediately fails?

Answer: Control backtracks to the most recent choice point or the call fails

A failing predicate causes backtracking; if no choice point exists, the enclosing goal fails.