[Ironruby-core] object equality

Tomas Matousek Tomas.Matousek at microsoft.com
Fri Sep 10 01:11:27 EDT 2010


There is indeed a typo… the last line should read:
Assert((bool)( equatable != 101));

Tomas

From: Tomas Matousek
Sent: Thursday, September 09, 2010 10:10 PM
To: ironruby-core at rubyforge.org
Subject: RE: [Ironruby-core] object equality

If any of the operands of == or != are typed to dynamic C# emits a call to dynamic site with the corresponding operation. If the left hand side is a Ruby object then this dynamic operation is translated to a Ruby method call “==” or “!=”, respectively.
The latest version of IronRuby (built from github sources) is compatible with MRI 1.9.2 in the way “!=” method is implemented: BasicObject#!= calls == and negates the result. You can also provide your own implementation of != method if its behavior should be different. This semantics is compatible with DLR dynamic operations and everything works as expected:

dynamic equatable = Engine.Execute(@"
class RubyEquatable
    def initialize val
      @val = val
    end

    def ==(other)
      @val == other
    end
end

RubyEquatable.new(100)
");

Assert((bool)( equatable == 100));
Assert(!(bool)( equatable == 101));
Assert(!(bool)( equatable != 100));
Assert((bool)( equatable != 100));

Tomas

From: ironruby-core-bounces at rubyforge.org [mailto:ironruby-core-bounces at rubyforge.org] On Behalf Of Davy Brion
Sent: Thursday, September 09, 2010 4:46 AM
To: ironruby-core at rubyforge.org
Subject: Re: [Ironruby-core] object equality


On Thu, Sep 9, 2010 at 1:44 PM, Davy Brion <ralinx at davybrion.com<mailto:ralinx at davybrion.com>> wrote:
by default, == does a reference check in C#, unless you override it to do a value based check (which you typically implement in Equals)

in C#, if you want == and != to work properly you need to implement them both.  In ruby, you obviously can't implement !=, but i had (naievely perhaps) expected that IronRuby would preserve the Ruby behavior when calling == on an object which implements it.  I _think_ that would be the best way to handle this, though there might be very valid reasons as to why this isn't the case at the moment.

defining an Equals method on the ruby class which delegates to == works, but it is somewhat weird since most people use the == and != operators to check for equality.

perhaps i'm better off getting rid of the == implementation and solely providing an Equals implementation


though the downside of that is that the ruby class doesn't stick to ruby's idioms when it comes to equality checks, which hurts its usage from other ruby code :s


On Thu, Sep 9, 2010 at 1:10 PM, William Green <will at hotgazpacho.org<mailto:will at hotgazpacho.org>> wrote:
Then it would appear that in C#, using the != operator on two instances of Ruby objects does not call the == method on the first Ruby object and invert the result.

Can you switch to using equals as a work-around?

Not sure of the semantics around == vs .Equals in C#, but I know there is a semantic difference between == and eql? in Ruby.

--
Will Green
http://hotgazpacho.org/



On Sep 9, 2010, at 6:52 AM, Davy Brion <ralinx at davybrion.com<mailto:ralinx at davybrion.com>> wrote:
the problem isn't with checking wether 2 objects are equal (though you indeed need to define an Equals method on your ruby object if you want the comparison to work with a direct call to .Equals... doing == in C# definitely uses the == method of your ruby object) but it is with the != check.  In ruby, using != calls == and inverts the result of that.  Doing != in C# on a ruby object doesn't seem to do the same thing.

I'm also not entirely sure how it _should_ be... but as far as i can tell, right now, i can't get equality checks working properly with ruby objects.

if a == b is true, then a != b should always be false
if a.Equals(b) is true, then !a.Equals(b) should always be false

if there's another way to get this behavior working in C# for ruby objects, i'd love to hear about it since it's pretty important for something i'm trying to do :)
On Thu, Sep 9, 2010 at 12:20 PM, William Green <will at hotgazpacho.org<mailto:will at hotgazpacho.org>> wrote:
Testing for object equality in C# is different than it is in Ruby. In
C#, you need to override both Object.Equals and Object.GetHashCode (I
forget which is used when, but I do recall that the compiler complains
if you override one and not the other). So, when you bring your Ruby
object into C# and compare them, C# doesn't see an override for Equals
on your object, and thus uses Object.Equals (which is often what you
don't want). Try defining an equals method on your Ruby object, or
alias it to ==.

I suspect that the == method on your Ruby object does not map to
Equals when you bring into C#. And I'm not sure that it should.

Thoughts?

--
Will Green
http://hotgazpacho.org/



On Sep 9, 2010, at 4:33 AM, Davy Brion <ralinx at davybrion.com<mailto:ralinx at davybrion.com>> wrote:

> If i have the following class in ruby:
>
> class TestClass
>   def initialize(value)
>     @value = value
>   end
>
>   def ==(other)
>     return false if other.nil?
>     self.value == other.value
>   end
>
>   protected
>
>   def value
>     @value
>   end
> end
>
> test1 = TestClass.new(5)
> test2 = TestClass.new(5)
> p test1 == test2
> p test1 != test2
>
> the output is:
> true
> false
>
> if i do this in .NET:
>
> dynamic test1 = ruby.TestClass. at new(5)<mailto:ruby.TestClass. at new(5)>;
> dynamic test2 = ruby.TestClass. at new(5)<mailto:ruby.TestClass. at new(5)>;
>
> var equals = test1 == test2;
> var differs = test1 != test2;
>
> both equals and differs are true
>
> i'm going to create an issue about this, but i do need to get this working... is there a temporary workaround that i can use for now?
> _______________________________________________
> Ironruby-core mailing list
> Ironruby-core at rubyforge.org<mailto:Ironruby-core at rubyforge.org>
> http://rubyforge.org/mailman/listinfo/ironruby-core
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