Scala varargs to Java varargs interop problem

On Tue, Apr 28, 2009 at 10:36:13AM -0500, Nils Kilden-Pedersen wrote:
> class TestVarArgs {
>
> def printIt(string: String, parms: Any*) {
> java.lang.System.out.printf(string, parms);
> }
>
> def main(args: Array[String]) {
> val test = new TestVarArgs
> test.printIt("%d, %d, %d", 1, 2, 3)
> }
> }

The parms arrives as a Seq, it doesn't keep acting "varargy" when you
pass it along. What you would want is:

def printIt(string: String, parms: Any*) {
java.lang.System.out.printf(string, parms: _*);
}

However this doesn't work since you're passing Anys and they're not
boxed, so I had to do this:

def printIt(string: String, parms: Any*) {
java.lang.System.out.printf(string,
parms.map(_.asInstanceOf[AnyRef]) : _*);
}

Not too pretty but I don't know a cleaner way at present.

The parms arrives as a Seq, it doesn't keep acting "varargy" when you
pass it along.  What you would want is:

   def printIt(string: String, parms: Any*) {
       java.lang.System.out.printf(string, parms: _*);
   }

However this doesn't work since you're passing Anys and they're not
boxed, so I had to do this:

   def printIt(string: String, parms: Any*) {
       java.lang.System.out.printf(string,
         parms.map(_.asInstanceOf[AnyRef]) : _*);
   }

Not too pretty but I don't know a cleaner way at present.

On Fri, May 1, 2009 at 6:37 AM, Nils Kilden-Pedersen wrote:
> On Tue, Apr 28, 2009 at 12:07 PM, Paul Phillips wrote:
>>
>> The parms arrives as a Seq, it doesn't keep acting "varargy" when you
>> pass it along.  What you would want is:
>>
>>    def printIt(string: String, parms: Any*) {
>>        java.lang.System.out.printf(string, parms: _*);
>>    }
>>
>> However this doesn't work since you're passing Anys and they're not
>> boxed, so I had to do this:
>>
>>    def printIt(string: String, parms: Any*) {
>>        java.lang.System.out.printf(string,
>>          parms.map(_.asInstanceOf[AnyRef]) : _*);
>>    }
>>
>> Not too pretty but I don't know a cleaner way at present.
>
> Are there any plans for Scala to support seamless interoperability with Java
> varargs? Or am doing something wrong?
>
It's on our to-do list for 2.8. We could not change it before, because
such a change will break binary compatibility.

Cheers

It's on our to-do list for 2.8. We could not change it before, because
such a change will break binary compatibility.

On Fri, May 01, 2009 at 12:38:40PM +0200, martin odersky wrote:

>> Are there any plans for Scala to support seamless interoperability with Java
>> varargs? Or am doing something wrong?
>>
>It's on our to-do list for 2.8. We could not change it before, because
>such a change will break binary compatibility.

RichString's format, although kind of ugly, seems to work reasonably
well, for formatting operations.

"%d, %d, %d".format(1,2,3)

or

"%d, %d, %d" format (1,2,3)

And, building upon that, things such as printf are fairly easy:

def printf(text: String, args: Any*) { print(text format (args : _*)) }

which is already defined in Predef (but not quite this way).

Calling the Java varargs ones is harder, because of the ambiguous
boxing. But, with some help it works, although annoying:

String.format("%d, %d", int2Integer(1), int2Integer(2))

David