Filter alist keys


You have an association list and you want a copy with only the specified keys.


Assuming no duplicate keys

(define (match value)
  (lambda (x)
    (equal? x value)))

(define (alist->subset keys alist)
  (let iter ((alist alist) (keys keys) (result '()))
    (if (or (null? alist) (null? keys))
        (reverse result)
        (let* ((pair (car alist)) (key (car pair)))
          (if (member key keys)
              (let ((keys (remove (match keys) keys)))
                 (iter (cdr alist) keys (cons pair result)))
              (iter (cdr alist) keys result))))))

Credit: Jakub T. Jankiewicz

It uses the remove procedure from another recipe.

Handling duplicate keys

(define (alist->subset keys alist)
  (let loop ((alist alist) (new-alist '()))
    (if (null? alist) (reverse new-alist)
        (loop (cdr alist)
              (let ((pair (car alist)))
                (if (member (car pair) keys)
                    (cons pair new-alist)))))))

Credit: Lassi Kortela


(define alist '((foo . 10) (bar . 20) (baz . 30) (quux . 40)))

(alist->subset '(quux foo) alist)
;; ==> ((foo . 10) (quux . 40))

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