Question/suggestion for hash-table page [from sourceforge / feature requests] How to use CLOS objects as hash-table keys? about cl-cookbook

[pnathan]

CDR-2 covers it. genhash is an available library for this.

[vindarel]

https://www.reddit.com/r/Common_Lisp/comments/es7wso/the_best_way_to_use_clos_objects_as_keys_for/

Recently I've (re-)discovered that you cannot (successfully) use CLOS objects as keys for hash-tables.
This is due to the fact that hash-tables only allow standard comparators (eq/eql/equal(p)). These do not work for CLOS objects and you cannot provide your own comparator. There is a genhash library that I am going to use now, but I don't like using external libraries for such simple problems.

i recently had a pleasant experience with generic-cl. i didn't want to use the whole thing (or rather, i saw some weird errors when i passed (:mix :some-package-that-shadows-a-cl-symbol :generic-cl) to uiop:define-package and got scared off), so i just did (:shadowing-import-from :generic-cl :equalp :hash-map :make-hash-map :get :hash). then i defined equalp and hash methods. for a class named -> with slots input and output, i have:

(defmethod equalp ((lhs ->) (rhs ->))
  (and (equalp (->-input lhs) (->-input rhs))
       (equalp (->-output lhs) (->-output rhs))))

(defmethod hash ((obj ->))
  (logand (+ (hash (->-input obj))
             (hash (->-output obj)))
          most-positive-fixnum))

from there, i just find & replace'd hash-table => hash-map and gethash => get.

[vindarel]

But it's too uncommon isn't it?