A Look at Failing Polymorphism

First, a brief recap of Steve Yegge's "When Polymorphism Fails" in case it moves again. Let's skip to the part where he builds an example in which polymorphism doesn't help the programmer, using a user-extensible online game as the premise.  A user wants to add an OpinionatedElf monster; how does she do it?

Let's say the OpinionatedElf's sole purpose in life is to proclaim whether it likes other monsters or not. It sits on your shoulder, and whenever you run into, say, an Orc, it screams bloodthirstily: "I hate Orcs!!! Aaaaaargh!!!" (This, incidentally, is how I feel about C++.)

The polymorphic approach to this problem is simple: go through every one of your 150 monsters and add a doesMrOpinionatedElfHateYou() method.

Gosh. That sounds insanely stupid.

Yegge then compares that to an alternative, implemented within the OpinionatedElf (who is the one doing the judgement in the first place):

public boolean doesElfLikeIt ( Monster mon )
{
    if ( mon instanceof Orc ) { return false; }
    if ( mon instanceof Elf ) { return true; }
    ... <repeat 150 times>
}

There's also the viable-in-Ruby approach of opening all the classes and adding the doesElfLikeIt method to each of the 150 monsters. But still, there's a lingering problem with all of the approaches so far:

If someone adds a Gremlin, your elf will be stuck screaming something like "Gosh, what's THAT?" until you can update his code to include a case for gremlins.

I ran into my own instance of this problem recently.

Labels

I figured out my underlying problem with Yegge's liberal/conservative (libertarian/authoritarian) division of programming cultures.

People like looking down on those considered inferior.  "Conservative" adds another way to do just that.

Radical Clojure

Apparently, last month I missed Yegge's post and followup regarding software liberals and conservatives.

One of the things that caught my eye, that's clearly a Nerd Trap but so powerful that I need to answer anyway, is this little quote:

But the reality is that Clojure is quite conservative.  As far as I'm concerned, if you can't set the value of a memory variable without writing code to ask for a transaction, it's... it's... all I can think of is Morgan Freeman in the Shawshank Redemption, working at the grocery store, complaining "Forty years I been asking permission to piss. I can't squeeze a drop without say-so."

Emphasis and vivid FUD in original.

There's just one missing word, though, that makes all the difference:

You can't set the value of a shared memory variable outside of a transaction.

Shared.  Global.  Possibly still in use by someone else.

Clojure is utterly pointless without understanding that time is explicit and everything is an immutable value (unless you have a Java native thing.)  Values last, unmodified, until a reader asks for an update, and so a writer must be forbidden from modifying (destroying) that memory.  There's a whole paradigm hiding there, which you can see again if you look at Datomic.  Clojure without immutable values might as well be JRuby.

The other trick up Clojure's sleeve is that 'transaction' implies something rather more heavyweight outside of Clojure than in it.  Again, because immutability is pervasive, Clojure's transactions don't have to do read tracking.  When anyone reads an old value, it's going to stay what they read.  It's like RCU, conceptually, except that every read is protected and nobody needs to copy because the messy details are taken care of on write.

And if it's not actually shared?  Then you should use a var, which only one thread can see, and therefore doesn't need to be updated in a transaction!

Clojure is an interesting language.  I'd still recommend it.


Added 26 Sept: I think that liberal/conservative as applied by Yegge divide the languages according to how much a language insists on its own philosophy.  Things like Erlang and Clojure get ranked "conservative" right in with Pascal because they have Ways to Do Things, even if they are progressive ideas (functional and parallel/async are strongly encouraged.)  Then Perl is more "liberal" because it's equally well-suited to OO, procedural, functional, and concatenative programming, i.e. just barely, and if it didn't have regexes as syntax, it would be long dead already.  Python is more conservative than Ruby despite sharing characteristics because "there should be one—and preferably only one—obvious way to do it."  That's a clear Conservative value, right in the middle of the Zen of Python.

In any case, the insight that everyone thinks they're liberal is accurate.  Upon some thinking, which is hard and time-consuming and therefore not generally applied on the Internet, "conservative" is a fair enough label for my tendencies at this point in time—because I got burned throwing around too much fire, then came off that job a few months later to take positions in more conservative cultures.  But I've never thought of myself as a stodgy dinosaur programmer.