Common Lisp predicate basics: which function returns t when and only when the provided object is a symbol?

Difficulty: Easy

(*

Question

Solve this multiple-choice question and choose the correct option.

Accepted Answer

Introduction / Context: Lisp code frequently inspects data types at runtime. Recognizing the correct predicate for symbols is fundamental, especially when manipulating code-as-data, macros, or abstract syntax trees where symbols serve as identifiers. Given Data / Assumptions: The dialect is Common Lisp. Predicates often end with p and return t or nil. We need a general test for “is this value a symbol?” Concept / Approach: The standard predicate for symbols is symbolp. Alternatives listed either are not predicates of the right kind or test different properties: constantp checks whether a form is a compile-time constant, while (* ) is an arithmetic call, not a predicate. There is no standard nonnumeric predicate in Common Lisp. Step-by-Step Solution: Identify candidate predicates that match Common Lisp naming conventions.Recall that symbolp returns t for symbols and nil otherwise.Select (symbolp ). Verification / Alternative check: In a REPL: (symbolp 'x) → t; (symbolp 42) → nil. This confirms the behavior. Why Other Options Are Wrong: (* ): multiplication operator, not a type test.(nonnumeric ): not a standard CL predicate.(constantp ): tests constant forms, not symbol-ness.None of the above: incorrect because symbolp is correct. Common Pitfalls: Confusing symbol tests with string or keyword tests; remember keywords are symbols too. Final Answer: (symbolp )

Common Lisp predicate basics: which function returns t when and only when the provided object is a symbol?

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