Assv

Assv is a standard function in Common Lisp used to locate a key in an association list (alist) by applying the eqv predicate. An alist is a list of cons cells where each car is a key and each cdr is the corresponding value. The function returns the first cons cell whose car is eqv to the given key, or nil if no such pair exists. It is one of several related functions for alists, with assoc using a customizable test (defaulting to eql), assq using eq, and assv using eqv.

Syntax and behavior: The typical usage is (assv key alist). If a matching pair is found, the function returns that cons cell (for example, (key . value)); otherwise, it returns nil. To obtain the value directly, one can take the cdr of the result, as in (cdr (assv key alist)).

Examples: Suppose the alist is ((a . 1) (b . 2) (c . 3)).

- (assv 'b '((a . 1) (b . 2) (c . 3))) yields (b . 2).

- (cdr (assv 'b '((a . 1) (b . 2) (c . 3)))) yields 2.

Because eqv considers certain objects as equivalent, assv is particularly suitable when keys are symbols or other objects with eqv semantics.

Compatibility and usage: assv is defined in ANSI Common Lisp and is supported by major implementations such as SBCL, CLISP, and Clozure CL. It is less common in Scheme, where similar functionality is provided by different procedures or may not be standardized. When working with alists in languages that lack assv, the same behavior can often be achieved with a combination of a custom predicate or by using assoc with an appropriate test. See also: assoc, assq, rassoc, rassv.