What's the value of case class X(x:Int)(y:String) vs case class X(x:Int, y:String)?

[Jacek Laskowski]

Hi,

I've been running into it quite often, but can't find the answer - why would one use two or more constructor's parameter lists over one? When is case class X(x:Int)(y:String) better than case class X(x:Int, y:String)? I'd highly appreciate samples of their use cases.

Jacek

[Konrad Malawski]

Hello Jacek, you've made me to register here so I could reply ;-)

To start out, I've never seen anyone using a case class with multiple parameter lists.

Back to what it actually does, first the normal syntax:

scala> case class A(a: Int, b: String)

scala> :javap A
Compiled from "<console>"
public class A extends java.lang.Object implements scala.Product,scala.Serializable{
    public int a(); // a val
    public java.lang.String b(); // b val

    public A copy(int, java.lang.String); 
    public int copy$default$1();
    public java.lang.String copy$default$2();
    public java.lang.String productPrefix();
    public int productArity();
    public java.lang.Object productElement(int);
    public scala.collection.Iterator productIterator();
    public boolean canEqual(java.lang.Object);
    public int hashCode();
    public java.lang.String toString();
    public boolean equals(java.lang.Object);

    public A(int, java.lang.String); // classic constructor, expected
}

And let's investigate what your ()() syntax does to a case class:

scala> case class B(a: Int)(b: String)
defined class B

scala> :javap B
Compiled from "<console>"
public class B extends java.lang.Object implements scala.Product,scala.Serializable{
    public int a(); // a val
    // no b value here

    public B copy(int, java.lang.String);
    public int copy$default$1();
    public java.lang.String productPrefix();
    public int productArity();
    public java.lang.Object productElement(int);
    public scala.collection.Iterator productIterator();
    public boolean canEqual(java.lang.Object);
    public int hashCode();
    public java.lang.String toString();
    public boolean equals(java.lang.Object);

    public B(int, java.lang.String); // classic constructor, expected
}

Well the constructor didn't change, yet you hid b using the second syntax...

Let's take a look at it's apply (B$ is the companion object of B):

scala> :javap B$
Compiled from "<console>"
public class B$ extends java.lang.Object implements scala.Serializable{
    public static final B$ MODULE$;
    public static {};
    public final java.lang.String toString();

    public B apply(int, java.lang.String); // still classic, as expected

So well... I don't see any reason why one would use the second syntax you proposed.

It makes a lot of sense for methods (it's for currying there), but definitely not for case classes - keep 'em simple :-)

Where did you bump into code using ()() in case classes? I can imagine using an implicit parameter list there, but otherwise I see no use case that appeals to me.