I'm just a simple property list, I didn't expect the Spanish strinquisition! (19 Jul 2007)

"Now, you're the Lisp aficionado here, right", he said, "You've got to help me out: Strings don't work in property lists!"

Oh, great. Who knows, being regarded (undeservedly) as the local Lisp, ahem, expert may become a factor for my job security some day, so I thought I'd better keep a straight face. Besides, he was still standing close to that window, and I wanted to leave nothing but a reassuring impression on him.

On the other hand, what the heck was he talking about?

Frantically grepping my grey cells for information on property lists, I somehow recalled we sometimes use them as a poor man's hashtable, usually mapping keywords to flags. But it had been so long I used property lists myself that I even had to look up the syntax details. To avoid this embarrassment next time around, here are some notes.

A property list is associated with a symbol. This flat and unstructured list can be thought of as a sequence of indicator/value pairs, with the indicator being the "key", in hash map terms. So the list starts with an indicator, followed by a value, followed by an indicator and its value, and so on. This is how you usually set a symbol property:

  (setf (get 'some-symbol some-indicator) some-value)

And to inquire a symbol property, you just say something like (get 'some-symbol some-indicator).

some-indicator can basically be any type, and so I wasn't sure what my co-worker meant when he said that he couldn't get strings to work, until he explained the details to me: He was calling some Lisp-based API function in our product, and that function returns a property list. Unfortunately, that property list was special in that somebody had stuffed a string into it as an indicator, and so the property list looked somehow like this:

  ("foo" 42 "bar" 4711)

And indeed, if you now try to inquire the "foo" property using (get 'some-symbol "foo"), all you get is - nil.

To retrieve a property value, get walks the list and compares each indicator in the list with "foo" (in this example) - using eq. From which we can immediately conclude:

• The correct spelling of "property list" is p-e-r-f-o-r-m-a-n-c-e p-r-o-b-l-e-m, as each lookup requires traversing potentially all of the list.
• eq checks for object equality, not just value equality. Which means that things like literal (!) strings or characters cannot be indicators!

In our case, we say (get 'some-symbol "foo"), and that "foo" string literal creates a new string object. While that new object happens to have the same value as the "foo" string in the property list, it is not the same object.

Indeed, the Common Lisp HyperSpec is quite clear on that topic: "Numbers and characters are not recommended for use as indicators in portable code since get tests with eq rather than eql, and consequently the effect of using such indicators is implementation-dependent."

It all boils down to the simple fact that (eq "foo" "foo") returns nil.

Now hopefully we can fix the API which returned those inadequate property lists to my co-worker's code, but his code also needs to run in older and current installations, and so he needed a workaround of some sort.

His first idea was to get the property list and fix it up in a preprocessing step before using get or getf for lookup, i.e. something like this:

(defun fix-plist(plist old-indicator new-indicator)
  (let ((cnt 0))
    (mapcar 
      #'(lambda(item)
          (incf cnt)
          (if (and (oddp cnt) (equal item old-indicator))
              new-indicator item))
      plist)))

(setf my-symbol 42)
(setf (get 'my-symbol "indicator") "value") ;; mess up plist
(print (get 'my-symbol "indicator"))        ;; returns NIL
(print (getf (fix-plist (symbol-plist 'my-symbol) "indicator" :indicator) :indicator))

This works, kind of - but it is actually quite ugly. Sure, with this code, we should be able to safely move ahead, especially since I also closed that office window in the meantime, but still: I really hope I'm missing something here. Any other ideas out there?

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