Thursday, May 23, 2013

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.