Re: Any way to collect all the values of a hash table more concisely ?

So this new version works as intended:

 

(defun cluster-by (fn list &aux (clusters (make-hash-table)) (result nil))
    (mapcar #'(lambda (x) (push x (gethash (funcall fn x) clusters))) list)
    (maphash  #'(lambda (key value) (push value result)) clusters)
    (sort result #'< :key #'(lambda (x) (length (car x)))))

 
Um, sorry:
 
(defun cluster-by (fn list &aux (clusters (make-hash-table)) (result nil))
     (mapcar #'(lambda (x) (push x (gethash (funcall fn x) clusters))) list)
     (maphash  #'(lambda (key value) (push value result)) clusters)
     (sort result #'< :key #'(lambda (x) (funcall fn (car x)))))
                                          ^^^^^^^^^^

"lambda" is a macro that expands to (function (lambda ...)).
So instead of
  #'(lambda
he ought to use
  (lambda

The poster prefers #'lambda because he prefers ugliness.

Common Lisp is used by those who want code to be as
hideous as possible.

Gauche Scheme

(define (cluster-by fn lst)
  (let ((clusters (make-hash-table 'equal?)))
    (dolist (k lst)
      (hash-table-push! clusters (fn k) k))
    (hash-table-values clusters)))

(cluster-by string-length '("a" "b" "abc" "bc" "a" "abcd" "e" "fg"))
  ===>
(("abcd") ("e" "a" "b" "a") ("fg" "bc") ("abc"))