Re: Comparing hash table objects

[Ihor Radchenko]

Richard Stallman <rms@gnu.org> writes:

> It clearly can be useful to compare hash tables for equivalence
> of contents.  I see two ways to offer that facility:
>
> * As a new function.  That would be upward-compatible.

I have seen various variants of such function in third-party packages
and just in internet search results.
For example, ht-equal? from
https://github.com/Wilfred/ht.el/blob/master/ht.el:

(defun ht-equal? (table1 table2)
  "Return t if TABLE1 and TABLE2 have the same keys and values.
Does not compare equality predicates."
  (declare (side-effect-free t))
  (let ((keys1 (ht-keys table1))
        (keys2 (ht-keys table2))
        (sentinel (make-symbol "ht-sentinel")))
    (and (equal (length keys1) (length keys2))
         (--all?
          (equal (ht-get table1 it)
                 (ht-get table2 it sentinel))
          keys1))))

However, it will not help with the problem of comparing objects
containing hash tables. Unless those obejcts also define special
comparison function (which is inconvenient).

> * By making `equal' compare them that way.  That would fit the spirit
> of `equal', but could break some existing uses of `equal'.

I'd say that using `equal' on hash tables with the actual intention of
comparing tables by `eq' is calling for a trouble. At least, it is
misleading.

Best,
Ihor