Re: RFC: make maphash return a list

["Pascal J. Bourguignon" <pjb@informatimago.com>]

Wilfred Hughes <me@wilfred.me.uk> writes:

> I've been using Emacs hash tables a lot recently, and I would find
> several operations much easier if maphash returned a list. For
> example, I'd like to be able to do:
>
> (maphash (lambda (key value) key) some-table)
>
> to obtain the list of keys present in some-table. I would guess elisp
> is influenced by Common Lisp here, where maphash returns nil
> unconditionally. However, unlike elisp, Common Lisp's loop macro
> supports iterating over keys directly with (loop for key being
> hash-key of some-table ...) which helps considerably.
>
> I can't see any obvious way this would break backwards compatibility,
> so I threw together a trivial patch that makes maphash return a list
> of the results. I've attached the patch.
>
> Is there interest in this?

maphash is nice because it doesn't cons a list.

(setq lexical-binding t)
(defun hashmap (fun hash)
  "Returns a list of the results of fun called on each key value entry.
No defined order."
  (let ((result '()))
    (maphash (lambda (k v) (push (funcall fun k v) result)) hash)
    result))

(let ((h (make-hash-table)))
  (setf (gethash :u h) 1
        (gethash :d h) 2
        (gethash :t h) 3
        (gethash :q h) 4
        (gethash :c h) 5
        (gethash :s h) 6)
  (hashmap (function cons) h))
--> ((:s . 6) (:c . 5) (:q . 4) (:t . 3) (:d . 2) (:u . 1))

        

-- 
__Pascal Bourguignon__                     http://www.informatimago.com/
A bad day in () is better than a good day in {}.