chickadee » alist-lib » alist-update!

On analogy with hash-table-update!, descructively update an association.

alist

The alist to update

key

The key associated with the update

f

A monadic function taking the preëxisting key

thunk

The thunk to apply if no association exists

=

The equality predicate for keys

(define alist-update!
  (case-lambda
    ((alist key function)
     (alist-update!
       alist
       key
       function
       (lambda () (error "Key not found -- ALIST-UPDATE!" key))))
    ((alist key function thunk) (alist-update! alist key function thunk eqv?))
    ((alist key function thunk =)
     (let ((pair (assoc key alist =)))
       (if pair
         (set-cdr! pair (function (cdr pair)))
         (alist-set! alist key (function (thunk))))))))