5.6.11.2 Adding or Setting Alist Entries

acons adds a new entry to an association list and returns the combined association list. The combined alist is formed by consing the new entry onto the head of the alist specified in the acons procedure call. So the specified alist is not modified, but its contents become shared with the tail of the combined alist that acons returns.

In the most common usage of acons, a variable holding the original association list is updated with the combined alist:

     (set! address-list (acons name address address-list))

In such cases, it doesn't matter that the old and new values of address-list share some of their contents, since the old value is usually no longer independently accessible.

Note that acons adds the specified new entry regardless of whether the alist may already contain entries with keys that are, in some sense, the same as that of the new entry. Thus acons is ideal for building alists where there is no concept of key uniqueness.

     (set! task-list (acons 3 "pay gas bill" '()))
     task-list
     =>
     ((3 . "pay gas bill"))
     
     (set! task-list (acons 3 "tidy bedroom" task-list))
     task-list
     =>
     ((3 . "tidy bedroom") (3 . "pay gas bill"))

assq-set!, assv-set! and assoc-set! are used to add or replace an entry in an association list where there is a concept of key uniqueness. If the specified association list already contains an entry whose key is the same as that specified in the procedure call, the existing entry is replaced by the new one. Otherwise, the new entry is consed onto the head of the old association list to create the combined alist. In all cases, these procedures return the combined alist.

assq-set! and friends may destructively modify the structure of the old association list in such a way that an existing variable is correctly updated without having to set! it to the value returned:

     address-list
     =>
     (("mary" . "34 Elm Road") ("james" . "16 Bow Street"))
     
     (assoc-set! address-list "james" "1a London Road")
     =>
     (("mary" . "34 Elm Road") ("james" . "1a London Road"))
     
     address-list
     =>
     (("mary" . "34 Elm Road") ("james" . "1a London Road"))

Or they may not:

     (assoc-set! address-list "bob" "11 Newington Avenue")
     =>
     (("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road")
      ("james" . "1a London Road"))
     
     address-list
     =>
     (("mary" . "34 Elm Road") ("james" . "1a London Road"))

The only safe way to update an association list variable when adding or replacing an entry like this is to set! the variable to the returned value:

     (set! address-list
           (assoc-set! address-list "bob" "11 Newington Avenue"))
     address-list
     =>
     (("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road")
      ("james" . "1a London Road"))

Because of this slight inconvenience, you may find it more convenient to use hash tables to store dictionary data. If your application will not be modifying the contents of an alist very often, this may not make much difference to you.

If you need to keep the old value of an association list in a form independent from the list that results from modification by acons, assq-set!, assv-set! or assoc-set!, use list-copy to copy the old association list before modifying it.