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Friday, December 28, 2007
Something in Ruby that bit me at work today
The methods :class and :dup are a bit of a leaky abstraction when used in combination with an object’s Singleton Class:
The methods :class and :dup are a bit of a leaky abstraction when used in combination with an object’s Singleton Class:
class Marx
def jokes (laughs)
laughs * 2
end
end
groucho = Marx.new
chico = groucho.dup
puts "#{groucho.jokes(10)} == #{chico.jokes(10)}"
def groucho.jokes (laughs)
laughs / 2
end
harpo = groucho.dup
puts "#{groucho.jokes(10)} != #{harpo.jokes(10)}"
puts "...but #{harpo.jokes(10)} == #{chico.jokes(10)}"
puts "because Groucho is still considered a #{groucho.class}"
Comments on “Something in Ruby that bit me at work today”:
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This gets me now and then in Ruby. It's one of those things that, once you understand it, it makes sense, but before you understand it you wouldn't immediately intuit it.
At first it feels like a POLS violation (Principle of Least Surprise) but then you see that it could go two ways and there's no clear way to know beforehand which one is right--and often that the way that bit you makes more sense coming from the "language up" rather than from the "problem down".
I call this a POOY violaton. ("This is clearly a violation of the Principle Of... Oh... Yeah.")
Edit: Now with 33% more correctly acronymized letters!
At first it feels like a POLS violation (Principle of Least Surprise) but then you see that it could go two ways and there's no clear way to know beforehand which one is right--and often that the way that bit you makes more sense coming from the "language up" rather than from the "problem down".
I call this a POOY violaton. ("This is clearly a violation of the Principle Of... Oh... Yeah.")
Edit: Now with 33% more correctly acronymized letters!
Yes, that's the desired behavior of Ruby IIRC. And if you want the singleton class be copied, you need to use clone instead of dup.o
Redefining methods on namespaces is complicated; as other commenters have mentioned, you can't consider this a violation of least surprise because no one quite knows what to expect.
It's nice if certain namespaces can't change in a given context. Ruby's :class and :dup both more or less assume invariance - and it's kinda hard to see what those should do if that invariant is broken.
If you restrict your redefinitions to a 'preamble', for example of a language extension/library (like say, rails), or for your own project, you don't usually run into these problems.
It's nice if certain namespaces can't change in a given context. Ruby's :class and :dup both more or less assume invariance - and it's kinda hard to see what those should do if that invariant is broken.
If you restrict your redefinitions to a 'preamble', for example of a language extension/library (like say, rails), or for your own project, you don't usually run into these problems.
If you restrict your redefinitions to a 'preamble', for example of a language extension/library (like say, rails), or for your own project, you don't usually run into these problems.
Except, even in a preamble this will be a problem. It doesn't matter where you bind methods to an object, the default dup method will never create objects with your methods, because dup works from the class of the object, and the class does not capture the singleton class.
I guess the real gotcha here is that objects have singleton classes, but that the .class method does not return them, and that methods like dup are tied to the class and not the singleton class.
Except, even in a preamble this will be a problem. It doesn't matter where you bind methods to an object, the default dup method will never create objects with your methods, because dup works from the class of the object, and the class does not capture the singleton class.
I guess the real gotcha here is that objects have singleton classes, but that the .class method does not return them, and that methods like dup are tied to the class and not the singleton class.
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