"Mapping CLOS", or "Yak Shaving for Fun and No Profit"

Sat Sep 24, 2011

Just a quick update today; I have more work than I think I can handle for the foreseeable future. A combination of problems I've never solved at work, some odd contract requests and several projects I've gotten myself into in my personal time. You already know about the CLSQL/Hunchentoot crash course, and the clomments system. You probably don't know about the last two, one of which sort of chains off of clomments1.

The background is that I wanted conversation threads in clomments. As in, when you see a comment, you should be able to reply to it. You can store that kind of thing in a relational database, but it feels a hacky. Especially if you want the system to scale out. I've looked at a couple of nosql databases in the past and have been looking for an excuse to use them in practice. Well, it turns out that storing hierarchical data is relatively simple in them. I picked CouchDB for no particular reason2, and tried out a few things. Unfortunately, clouchdb has at least one [annoying bug](http://stackoverflow.com/questions/7537018/clouchdb-error-with-id) that prevents me from using it. It wouldn't normally, I've already sent a one-liner patch to the author, but he's currently working at one of these Disney-style-IP places that own your intellectual property, DNA and all derivative works of both (I'm paraphrasing), so he can't actually merge that patch any time soon.

Ok, so I guess I'm using chillax then, which kind of saddens me because I'd prefer a project that used the same naming convention as the project I'm using it for, but that's beside the point. In either case, interacting with Couch from CL is kind of a pain in the ass. It involves tons of alists and many traversals of same, which isn't horrible, but definitely not as pleasant as interacting through CLOS the way I have been in CLSQL. Looking around, there doesn't seem to be an ORM-style thing already built for Couch or Mongo, so I figured I'd try my hand at one as part of the clomments project. I don't have anything workable yet, I've just been playing around so far, but the first leg of research has turned up a rather annoying implementation detail that segues nicely into the title of this article.

Do you know how to map over a CLOS object?

If you've never thought about it before, feel free to go investigating. Hopefully, you have a better time than I did. If you just want to do it in SBCL, it's actually fairly simple

(defun slot-names (class)
  (mapcar #'sb-pcl:slot-definition-name
          (sb-pcl:class-slots class)))

(defun map-slots (fn instance)
  (loop for slot-name in (slot-names (class-of instance))
        collect (funcall fn slot-name (slot-value instance slot-name))))

The problem, as the astute among you have already noticed, is sb-pcl. That's actually an SBCL-only CLOS library that implements various functions found in the spec. Other lisps don't make the same decisions, so if you want to make these functions portable, you need to do some more work. Either going what I call "the insane route" (which I took some cues from) and conditionally define each function that deals with a class, or the clsql-sys route in which you don't specify a package for these functions, but rather :use different modules conditionally as part of your package definition. That looks like

(defpackage #:clos-utils
    (:use #:common-lisp)
   #+openmcl-native-threads #:ccl
   #+cmu #:pcl
   #+sbcl #:sb-pcl
   #+lispworks #:hcl
   #+allegro #:mop
   #+clisp #:clos
   #:class-slots #:slot-definition-name))

(in-package :clos-utils)

(defun slot-names (class)
  (mapcar #'slot-definition-name
          (class-slots class)))

(defun map-slots (fn instance)
  (loop for slot-name in (slot-names (class-of instance))
        collect (funcall fn slot-name (slot-value instance slot-name))))

That's still not fully portable, by the way. openmcl apparently calls class-slots class-instance-slots if it doesn't have openmcl-native-threads. But it's reasonably close to portable. Having come down off a week or so of Python/Ruby scripting, that...was a lot more work than I expected to do for a task like this. Hopefully this saves someone else out there some time (or causes someone to contact me, pointing out a much easier way of doing it). Anyhow, that's phase one of creating CLOS-based bindings for CouchDB, which will then let me succinctly work on clomments which should eventually increase the amount of Freedom on the net by some small increment.

  1. The other I'm conveniently ignoring for the duration of this post, except for this sentence.
  2. quicklisp has support for cl-mongo, clouchdb and chillax.

Creative Commons License

all articles at langnostic are licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License

Reprint, rehost and distribute freely (even for profit), but attribute the work and allow your readers the same freedoms. Here's a license widget you can use.

The menu background image is Jewel Wash, taken from Dan Zen's flickr stream and released under a CC-BY license