How to define and invoke a partially applied function with varargs?

Question

Stack Over Flow:

Hi all.

I would like to define a partially applied function which has varargs.

To confirm, I prepared several functions i.e. Functions._. The functions except func have a varargs argument.

How to define and invoke a partially applied function with varargs? (e.g. gunc1 with varargs.)

java version "1.7.0_07" Scala code runner version 2.9.1.final

object Functions {
    def func(x: Int, y: Int) = x + y
    def gunc(x: Int*) = x.sum
    def hunc(x: Int, y: Int*) = x + y.sum
    def iunc(x: Int)(y: Int*) = x + y.sum // curried
    def junk(x: String, y: Int*) = x + y.sum
}

object PartiallyApplied extends App {
    import Functions._

    val func0 = func(1, _: Int) // I can.
    println("result: " + func0(2))

    val gunc0 = gunc(1, _: Int) // I can invoke if I specify the varargs.size. But gunc0 no longer has varargs...
    println("result: " + gunc0(2))

//    val gunc1 = gunc(1, _: Int*) => compile error: ')' expected but identifier found.
//    val gunc1 = gunc(1, _: Seq[Int]) => compile error: type mismatch | found: Seq[Int] | required: Int
    val gunc1 = gunc(1, _: Int) // I can invoke if I specify the varargs.size. But gunc1 no longer has varargs...
    println("result: " + gunc1(2))

//    val hunc0 = hunc(1)_ => compile error: _ must follow method; cannot follow Int
//    val hunc0 = hunc(1, _: Int*) => compile error: ')' expected but identifier found.
//    val hunc0 = hunc(1, _: Seq[Int]) => compile error: type mismatch | found: Seq[Int] | required: Int
    val hunc0 = hunc(1, _: Int) // I can invoke if I specify the varargs.size. But hunc0 no longer has varargs...
    println("result: " + hunc0(2))
//    println("result: " + hunc0(2, 3)) => compile error: too many arguments for method apply: (v1: Int)Int in trait Function1
//    println("result: " + hunc0((2, 3): _*)) => compile error: type mismatch | found: (Int, Int) | required: Seq[Int]
//    println("result: " + hunc0(Seq(2, 3))) => compile error: type mismatch | found: Seq[Int] | required: Int
//    println("result: " + hunc0(Seq(2, 3): _*)) => compile error: no `: _*' annotation allowed here (such annotations are only allowed in arguments to *-parameters)

    val hunc1 = hunc(1, _: Int, _: Int) // I can invoke if I specify the varargs.size. But hunc1 no longer has varargs...
    println("result: " + hunc1(2, 3))

    val hunc2 = hunc(1, _: Int, _: Int, _:Int) // I can invoke if I specify the varargs.size. But hunc2 no longer has varargs...
    val hunc3 = hunc2(2, _: Int, _: Int)
    val hunc4 = hunc3(3, _: Int)
    println("result: " + hunc4(4))

    println("result: " + hunc5(2, 3))

//    val iunc0 = iunc(1)(_: Int*) => compile error: ')' expected but identifier found.
//    val iunc0 = iunc(1)(_: Seq[Int]) => compile error: type mismatch | found: Seq[Int] | required: Int
    val iunc0 = iunc(1)(_: Int)
    println("result: " + iunc0(2))

    val iunc1 = iunc(1)(_: Int, _: Int)
    println("result: " + iunc1(2, 3))
}

object NotPartiallyApplied extends App {
    import Functions._

    println("result: " + gunc(1))
    println("result: " + gunc(1, 2, 3))
    println("result: " + gunc(Seq(1, 2, 3): _*))

    println("result: " + hunc(1))
    println("result: " + hunc(1, 2, 3))
    println("result: " + hunc(1, Seq(2, 3): _*))

    println("result: " + iunc(1)(2, 3))
    println("result: " + iunc(1)(Seq(2, 3): _*))

    println("result: " + junk("x"))
    println("result: " + junk("x", 2, 3))
    println("result: " + junk("x", Seq(2, 3): _*))
}

[Edit after Rex Kerr's comment]

I want the partially applied function like guncN below:

val guncN = gunc(1, _: Int*)
println("result: " + guncN(2)) // => 3
println("result: " + guncN(2, 3)) // => 6
println("result: " + guncN(2, 3, 4, 5, 100)) // => 115

But it is prohibited by scalac.

I think guncN should be evaluated each time, at time (2), (2, 3), and (2, 3, 4, 5, 100) are given. We don't need guncM below:

val guncM = guncN(2, _: Int*)
println("result: " + guncM(3, 4)) // => 10

Answer 1

If you just want varargs, you can in 2.9:

Welcome to Scala version 2.9.1.final
(Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_31).
...
scala>     def sum(xs: Int*) = xs.sum
sum: (xs: Int*)Int

scala>     val summer = sum _
summer: Int* => Int = <function1>

scala> summer(1,2,3)
res0: Int = 6

But not in 2.10 as it stands now, and there's no apparent plan to change this: https://issues.scala-lang.org/browse/SI-4176

Welcome to Scala version 2.10.0-RC2
(Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_31).
...

scala> def sum(xs: Int*) = xs.sum
sum: (xs: Int*)Int

scala> val summer = sum _
summer: Seq[Int] => Int = <function1>

scala> summer(1,2,3)
<console>:10: error: too many arguments for method apply:
(v1: Seq[Int])Int in trait Function1
              summer(1,2,3)

If you have 2.9, then you can

scala> def temp(early: Int*)(rest: Int*) = sum((early ++ rest): _*)
temp: (i: Int, j: Int)(rest: Int*)Int

scala> val presummed = temp(1,2) _
presummed: Int* => Int = <function1>

scala> presummed(4,5)
res1: Int = 12

but perhaps you shouldn't count on having varargs functions since that seems to have been deprecated (or at least moved behind the -Yeta-expand-keeps-star flag, which as a -Y option may fail to work or be removed at any time).

Answer 2

The solution I found is by composing functions :

Welcome to Scala version 2.10.0-RC1 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_37).
...
scala> 1 :: (_:Int) :: Nil : Seq[Int]
res42: Int => Seq[Int] = <function1>

scala> gunc _
res43: Seq[Int] => Int = <function1>

scala> res42 andThen res43
res54: Int => Int = <function1>

scala> res54(5)
res56: Int = 6