listp

listp is a function in some programming languages, particularly those in the Lisp family such as Common Lisp and Scheme. It is a predicate function, meaning it returns a boolean value, true or false. The primary purpose of listp is to check if its argument is a list.

When evaluated, listp takes a single argument. If the argument is a list, listp returns true. Otherwise, if the argument is not a list (e.g., a number, a symbol, a string, or a structure that is not a list), it returns false. This function is fundamental for data manipulation and control flow within Lisp-like environments, allowing programs to conditionally execute code based on the type of data structure encountered.

The definition of what constitutes a "list" can vary slightly between different Lisp dialects. In many implementations, listp evaluates to true for both proper lists (ending in nil) and dotted pairs, which represent the internal structure of lists. Understanding the precise behavior of listp in a specific programming environment is important for accurate programming. It's a common tool for type checking and ensuring that operations intended for lists are applied to appropriate data.