ASSOC.html -- ACL2 Version 6.3

### ASSOC

look up key in association list

    Major Section:  ACL2-BUILT-INS

    General Forms:
    (assoc x alist)
    (assoc x alist :test 'eql)   ; same as above (eql as equality test)
    (assoc x alist :test 'eq)    ; same, but eq is equality test
    (assoc x alist :test 'equal) ; same, but equal is equality test

`(Assoc x alist)` is the first member of `alist` whose `car` is `x`, or `nil` if no such member exists. The optional keyword, `:TEST`, has no effect logically, but provides the test (default `eql`) used for comparing `x` with the `car`s of successive elements of `alist`.

The guard for a call of `assoc` depends on the test. In all cases, the second argument must satisfy `alistp`. If the test is `eql`, then either the first argument must be suitable for `eql` (see eqlablep) or the second argument must satisfy `eqlable-alistp`. If the test is `eq`, then either the first argument must be a symbol or the second argument must satisfy `symbol-alistp`.

See equality-variants for a discussion of the relation between `assoc` and its variants:

> `(assoc-eq x alist)` is equivalent to `(assoc x alist :test 'eq)`;

> `(assoc-equal x alist)` is equivalent to `(assoc x alist :test 'equal)`.

In particular, reasoning about any of these primitives reduces to reasoning about the function `assoc-equal`.

`Assoc` is defined by Common Lisp. See any Common Lisp documentation for more information.