[alexandria-devel] Re: [PATCH] COPY-HASH-TABLE

[Nikodemus Siivola]

On Fri, Mar 14, 2008 at 3:51 PM, Tobias C. Rittweiler <tcr at freebits.de> wrote:

>  In case b): If the code frobs destructively, the destructive operation
>  is often not only restricted to the hash-table per se, but also to the
>  data structures contained in the hash-table.
>
>  Notice that COPY-HASH-TABLE is still shallow by default. It's just that
>  hash-tables are used in more complex situations, justifying a bit more
>  flexibility.

This is obviously a question of taste, but IMO in this case the deep
copying should be done by the frobber, not the copy-hash-table
operation. ...but my own main objection is that it is not too obvious
what :KEY means there.

Perhaps you might be better served with MAP-HASH-TABLE, which maps
into a new hash-table? Something like this (untested), maybe:

(defun map-hash-table (function table &rest more &key test size
rehash-size rehash-threshold)
  "Calls FUNCTION with every key and value in hash-table TABLE, and
saves the returned value under the same key in a new hash-table.
Finally returns the new hash-table. The new table has the same
properties as the original, unless overridden by the keyword
arguments. Implementation specific hash-table properties are not
copied by default, but can be specified using keywords."
  (setf test (or test (hash-table-test table))
        size (or size (hash-table-size table))
        rehash-size (or rehash-size (hash-table-size table))
        rehash-threshold (or rehash-threshold
(hash-table-rehash-threshold table)))
  (let ((real-function (ensure-function function))
        (copy (apply #'make-hash-table
                     :test test
                     :size size
                     :rehash-size rehash-size
                     :rehash-threshold rehash-threshold
                     more)))
    (maphash (lambda (k v)
               (setf (gethash k copy) (funcall real-function k v)))
             table)
    copy))

(My use-cases for COPY-HASH-TABLE are mainly for debugging: grab a
copy of an application internal hash-table at a certain point in time,
and then poke at it at leisure.)

Cheers,

 -- Nikodemus