chickadee » alist-lib » alist-ref

Return a value associated with its key or apply thunk.

alist

The alist to search in

key

The key whose value to return

thunk

The thunk to apply when association doesn't exist (default is to err)

=

The equality predicate to apply to keys

(define alist-ref
  (case-lambda
    ((alist key)
     (alist-ref
       alist
       key
       (lambda () (error "Key not found -- ALIST-REF" key))))
    ((alist key thunk) (alist-ref alist key thunk eqv?))
    ((alist key thunk =)
     (let ((value (assoc key alist =))) (if value (cdr value) (thunk))))))