Question

There is a class (specifically TypedPipe[T] in com.twitter.scalding) that has only a private constructor, and I wish to add some methods to it. It looks something like this

class TypedPipe[+T] private ( a : Int) { 

}

This is usually done by defining an implicit conversion method. However, I need access to a to define the method I want to. Is this simply impossible in Scala?

Answer 1

If I correctly interpreted your question, the answer is yes, this is simply impossible in Scala.
This has nothing do to with the private constructor. If it were public, you would have exactly the same problem.

class TypedPipe[+T] private ( a : Int) { 

}

The point here is that there is no val or var before a . a is not a property of TypedType, but only an argument to its constructor, i.e. no property is created to hold the value of a inside the object

This is equivalent to the Java code:

public class MyClass{
    private final String value;
    public MyClass(int a){
        System.out.println("I was initialized with " + a);
        this.value="b";
    }
}

and you can actually check it out easily with the REPL. If you want a to be accessible outside the constructor, it must either be a var or a val

class TypedPipe[+T] private ( a : Int) {
  println(a)
}

class TypedPipe1[+T] private (val a : Int) {
  println(a)
}

I am a big fan of the javap disassembler in the REPL:

scala> :javap -p TypedPipe

scala> Compiled from "TypedType.scala"
public class TypedPipe extends java.lang.Object implements scala.ScalaObject{
    private TypedPipe(int);
}


scala> :javap -p TypedPipe1

scala> Compiled from "TypedType.scala"
public class TypedPipe1 extends java.lang.Object implements scala.ScalaObject{
    private final int a;
    public int a();
    private TypedPipe1(int);
}

As you can see, in the first version there is no member property to hold the value of a, and no getter