From: Hal Fulton <hal9000@...>
Date: 2004-08-12T12:49:50+09:00
Subject: Interesting Ruby anomaly

Hello, all...

I ran across an interesting bit of behavior, and I'm not sure I
understand why it happens.

Here's the background. I have a SuperStruct class which I use frequently
(though I hope to find a better name for it). See the snippet on
rubyforge: http://rubyforge.org/snippet/detail.php?type=snippet&id=25

So. If you call SuperStruct.new, you get a class that is much like
a Struct; if you call SuperStruct.open instead, you get a class that
is more like an OpenStruct.

That is, in the latter case, if you reference a nonexistent attribute,
it will spring into existence:

   klass = SuperStruct.open(:alpha)  # Create with attr "alpha"
   foo = klass.new
   x = foo.beta      # nil
   foo.gamma = 5     # 5

In fact, when you referenced the getter OR the setter... then the getter
AND the setter would be defined. Make sense so far?

But this leads to an oddity...

   foo.delta ||= 7              # gives an error!
   foo.delta = foo.delta || 7   # I think this also gives an error

I had to fix this by saying: When the getter or setter is referenced,
define ONLY that method. I don't quite like this; but oh, well.

But puzzling out why it works this way has given me a headache.

What do you think?


Hal