Eq Type Class in Scala?
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Seyed H. HAERI (Hossein)
May 31, 2012, 6:57:32 AM5/31/12
to scala-user
Dear all,
Does Scala have anything like the Eq type class of Haskell which can
mitigate the big hassle of implementing the correct equals method for
case classes?
TIA,
--Hossein
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Seyed H. HAERI (Hossein)
Research Assistant
Institute for Software Systems (STS)
Technical University of Hamburg (TUHH)
Hamburg, Germany
ACCU - Professionalism in programming - http://www.accu.org/
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Does Scala have anything like the Eq type class of Haskell which can
mitigate the big hassle of implementing the correct equals method for
case classes?
TIA,
--Hossein
--------------------------------------------------------------------------------------------------------------
Seyed H. HAERI (Hossein)
Research Assistant
Institute for Software Systems (STS)
Technical University of Hamburg (TUHH)
Hamburg, Germany
ACCU - Professionalism in programming - http://www.accu.org/
--------------------------------------------------------------------------------------------------------------
Matthew Tyler
May 31, 2012, 7:06:04 AM5/31/12
to Seyed H. HAERI (Hossein), scala-user
Case classes provide the equals method. Is this insufficient for your requirements?
Seyed H. HAERI (Hossein)
May 31, 2012, 7:12:37 AM5/31/12
to scala-user
Hi Matthew,
> Case classes provide the equals method.
Oh, do they? And, that's structural equivalence like Eq of Haskell?
> Case classes provide the equals method.
Matthew Tyler
May 31, 2012, 7:14:01 AM5/31/12
to Seyed H. HAERI (Hossein), scala-user
I believe so.
Josh Suereth
May 31, 2012, 7:15:52 AM5/31/12
to Seyed H. HAERI (Hossein), scala-user
Yes. Case classes do implement both equals and hashCode by structure.
Seyed H. HAERI (Hossein)
May 31, 2012, 7:16:33 AM5/31/12
to scala-user
Cool! Would you mind referring me to the part of spec which says that?
--
Lukas Rytz
May 31, 2012, 7:23:03 AM5/31/12
to Seyed H. HAERI (Hossein), scala-user
end of 5.3.2
Seyed H. HAERI (Hossein)
May 31, 2012, 7:25:33 AM5/31/12
to scala-user
Great! Thanks. :)
Tony Morris
May 31, 2012, 7:28:56 AM5/31/12
to Seyed H. HAERI (Hossein), scala-user
scalaz.Equal
Josh Suereth
May 31, 2012, 7:32:45 AM5/31/12
to Tony Morris, scala-user, Seyed H. HAERI, (Hossein)
Except it doesn't have structural autogen yet...
Tony Morris
May 31, 2012, 4:49:18 PM5/31/12
to Josh Suereth, scala-user, Seyed H. HAERI, (Hossein)
Sure it does. It just looks a lot different and is flexible.
Equal composes.
Equal[A] => Equal[B] => Equal[B]
Combined with lenses, the effort required is proportional to the deviaton from the default. That is, the default requires no effort (is "generated").
Josh Suereth
May 31, 2012, 5:01:35 PM5/31/12
to Tony Morris, scala-user, Seyed H. HAERI, (Hossein)
Right. I guess I meant with less typing. Someday soon, I hope, typeclass will have less boilerplate in Scala. We have to wait for the new features to stabilize before adding more though....
Tony Morris
May 31, 2012, 5:11:25 PM5/31/12
to Josh Suereth, scala-user, Seyed H. HAERI, (Hossein)
If we are indulging in our desires of the language in this respect, I
might just add, in a rather loud voice, newtype/deriving.
--
Tony Morris
http://tmorris.net/
might just add, in a rather loud voice, newtype/deriving.
Tony Morris
http://tmorris.net/
Raoul Duke
May 31, 2012, 5:13:59 PM5/31/12
to scala-user
On Thu, May 31, 2012 at 2:11 PM, Tony Morris <tonym...@gmail.com> wrote:
> If we are indulging in our desires of the language in this respect, I
> might just add, in a rather loud voice, newtype/deriving.
yes, please, and how.
> If we are indulging in our desires of the language in this respect, I
> might just add, in a rather loud voice, newtype/deriving.
it always kills me that some such isn't obviously required in any
statically typed languages for it to be considered "real". :-}
Seyed H. HAERI (Hossein)
May 31, 2012, 6:58:25 PM5/31/12
to scala-user
Guys,
And, in case I want a different equality than the structural one, is
that possible to override equals for a case class?
TIA,
--Hossein
And, in case I want a different equality than the structural one, is
that possible to override equals for a case class?
TIA,
--Hossein
Grzegorz Balcerek
May 31, 2012, 10:13:50 PM5/31/12
to Seyed H. HAERI (Hossein), scala-user
Yes, it is.
scala> case class A(a: Int) { override def equals(x:Any) = true }
defined class A
scala> A(1) == A(2)
res0: Boolean = true
Grzegorz
scala> case class A(a: Int) { override def equals(x:Any) = true }
defined class A
scala> A(1) == A(2)
res0: Boolean = true
Grzegorz
missingfaktor
Jun 1, 2012, 1:10:05 AM6/1/12
to Seyed H. HAERI (Hossein), scala-user
And if you are using the type-class approach, just add a new implicit instance and have it in scope.
implicit object FooEqual extends Equal[Foo] {
def equal(a: Foo, b: Foo) = true
Tony Morris
Jun 1, 2012, 6:29:42 PM6/1/12
to Josh Suereth, scala-user, Seyed H. HAERI, (Hossein)
Correction.
Equal composes (to records).
Equal[A] => Equal[B] => Equal[(A,B)]
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