Warning: This is the manual of the legacy Guile 2.0 series. You may want to read the manual of the current stable series instead.

6.7.12.4 Removing Alist Entries

To remove the element from an association list whose key matches a specified key, use assq-remove!, assv-remove! or assoc-remove! (depending, as usual, on the level of equality required between the key that you specify and the keys in the association list).

As with assq-set! and friends, the specified alist may or may not be modified destructively, and the only safe way to update a variable containing the alist is to set! it to the value that assq-remove! and friends return.

address-list
⇒
(("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road")
 ("james" . "1a London Road"))

(set! address-list (assoc-remove! address-list "mary"))
address-list
⇒
(("bob" . "11 Newington Avenue") ("james" . "1a London Road"))

Note that, when assq/v/oc-remove! is used to modify an association list that has been constructed only using the corresponding assq/v/oc-set!, there can be at most one matching entry in the alist, so the question of multiple entries being removed in one go does not arise. If assq/v/oc-remove! is applied to an association list that has been constructed using acons, or an assq/v/oc-set! with a different level of equality, or any mixture of these, it removes only the first matching entry from the alist, even if the alist might contain further matching entries. For example:

(define address-list '())
(set! address-list (assq-set! address-list "mary" "11 Elm Street"))
(set! address-list (assq-set! address-list "mary" "57 Pine Drive"))
address-list
⇒
(("mary" . "57 Pine Drive") ("mary" . "11 Elm Street"))

(set! address-list (assoc-remove! address-list "mary"))
address-list
⇒
(("mary" . "11 Elm Street"))

In this example, the two instances of the string "mary" are not the same when compared using eq?, so the two assq-set! calls add two distinct entries to address-list. When compared using equal?, both "mary"s in address-list are the same as the "mary" in the assoc-remove! call, but assoc-remove! stops after removing the first matching entry that it finds, and so one of the "mary" entries is left in place.

Scheme Procedure: assq-remove! alist key

Scheme Procedure: assv-remove! alist key

Scheme Procedure: assoc-remove! alist key

C Function: scm_assq_remove_x (alist, key)

C Function: scm_assv_remove_x (alist, key)

C Function: scm_assoc_remove_x (alist, key)

Delete the first entry in alist associated with key, and return the resulting alist.