Sometimes it's convenient to store things in a list like '((key . value)
(another-key . value)) rather than a full blown (and unordered) hash
table.
It is available now in cl-irregsexp.utils but could you add the fairly
standard cdr-assoc function to alexandria?
(declaim (inline cdr-assoc))
(defun cdr-assoc (alist key &key (test 'eql))
(cdr (assoc key alist :test test)))
(define-setf-expander cdr-assoc (place key &key (test ''eql) &environment env)
(multiple-value-bind (dummies vals newval setter getter)
(get-setf-expansion place env)
(with-unique-names (store key-val test-val alist found)
(values
(append dummies (list key-val test-val))
(append vals (list key test))
`(,store ,@newval)
`(let (,@(mapcar 'list dummies vals)
(,alist ,getter))
(let ((,found (assoc ,key-val ,alist :test ,test-val)))
(cond (,found
(setf (cdr ,found) ,store))
(t
(setf ,(first newval) (acons ,key ,store ,alist))
,setter))
,store))
`(cdr-assoc ,getter)))))