assoc -- r7rs Definition

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Kind

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procedure;

Implemented by

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• assoc (from vonuvoli);

Procedure signature

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Procedure variants:

• ((any null) -> (false))
  • inputs:
    • a value of type any;
    • a value of type null;
  • output: a value of type false;
• ((any assoc-list-not-null) -> (list-not-null-or-false))
  • inputs:
    • a value of type any;
    • a value of type assoc-list-not-null;
  • output: a value of type list-not-null-or-false;
• ((any null procedure-2) -> (false))
  • inputs:
    • a value of type any;
    • a value of type null;
    • a value of type procedure-2;
  • output: a value of type false;
• ((any assoc-list-not-null procedure-2) -> (list-not-null-or-false))
  • inputs:
    • a value of type any;
    • a value of type assoc-list-not-null;
    • a value of type procedure-2;
  • output: a value of type list-not-null-or-false;

Exports

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• scheme:base -- (scheme base);

Exports recursive

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• scheme -- (scheme);

Description

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(assq obj alist)
(assv obj alist)
(assoc obj alist)
(assoc obj alist compare)

Domain: It is an error if alist (for association list) is not a list of pairs.

These procedures find the first pair in alist whose car field is obj, and returns that pair. If no pair in alist has obj as its car, then #f (not the empty list) is returned. The assq procedure uses eq? to compare obj with the car fields of the pairs in alist, while assv uses eqv? and assoc uses compare if given and equal? otherwise.

(define e '((a 1) (b 2) (c 3)))
(assq 'a e)               ===>  (a 1)
(assq 'b e)               ===>  (b 2)
(assq 'd e)               ===>  #f
(assq (list 'a) '(((a)) ((b)) ((c))))
                          ===>  #f
(assoc (list 'a) '(((a)) ((b)) ((c))))
                          ===>  ((a))
(assoc 2.0 '((1 1) (2 4) (3 9)) =)
                          ===> (2 4)
(assq 5 '((2 3) (5 7) (11 13)))
                          ===>  #unspecified
(assv 5 '((2 3) (5 7) (11 13)))
                          ===>  (5 7)

Rationale: Although they are often used as predicates, memq, memv, member, assq, assv, and assoc do not have question marks in their names because they return potentially useful values rather than just #t or #f.

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The text herein was sourced and adapted as described in the "R7RS attribution of various text snippets" appendix.

Referenced-types

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• any;
• null;
• false;
• assoc-list-not-null;
• list-not-null-or-false;
• procedure-2;