On: Five Dollar Programming Words

[a-priori]

His definition of idempotence is correct. As an example, a hash table with idempotent delete would work fine if I did this:

  (setf h (hash-new))
  (hash-insert h 'foo '(1 2 3 4))
  (hash-delete h 'foo)
  ;;; some time later
  (hash-delete h 'foo)
  

A non-idempotent implementation might complain that 'foo doesn't exist in the table.