Parameterless function identified as function with one Unit parameter

[gaeljw]

Minimized code

import scala.language.implicitConversions

final class MyClass(name: String) {

  final class Fun0(val f: Function0[Any])

  object Fun0 {

    implicit def function0AsFun0(f: Function0[Any]): Fun0 = new Fun0(f)

  }

  def apply(f: => Unit): Unit = {
    apply(() => f)
  }

  def apply(fun: Fun0): Unit = {
    // Do something
    println(s"Got a Fun0 $fun")
  }

  def apply[T1](f: (T1) => Any)(implicit m1: Manifest[T1]): Unit = {
    // Do something
    println(s"Got a Function1: ${f}")
  }

}

Output

12 |    apply(() => f)
   |                  ^
   |                  No Manifest available for Unit.

From what I understand the () => f is interpreted as Unit => Any rather than () => Any.

Expectation

This code compiles fine with Scala 2.13. Although I admit it's a bit twisted, I expected it to compile in Scala 3 as well.

Note that I found two workarounds:

// 1st workaround
def apply(f: => Unit): Unit = {
  apply((() => f)) // Notice the double parenthesis
}

// 2nd workaround
def apply(f: => Unit): Unit = {
  val fun = () => f
  apply(fun) 
}

This might be intended behavior and I can accept it easily but I prefer to post and ask in case this could be the indicator of a more serious issue.

[bishabosha]

This is different behaviour to Scala 2. In Scala 2 the implicit conversion to Fun0 is activated with apply(() => f)

[som-snytt]

The first two overloads are applicable, but the third is chosen in Scala 3.

There is a different error reported if renamed to avoid the overload:

➜  snips ~/scala3-3.0.0-M1/bin/scalac -d /tmp i10437d.scala
-- [E007] Type Mismatch Error: i10437d.scala:12:17 -----------------------------
12 |    apply3(() => f)
   |                 ^
   |                 Found:    () => Any
   |                 Required: Any => Any
-- Error: i10437d.scala:12:19 --------------------------------------------------
12 |    apply3(() => f)
   |                   ^
   |                   No Manifest available for Any.
2 errors found
➜  snips ~/scala3-3.0.0-M1/bin/scalac -d /tmp i10437.scala
-- Error: i10437.scala:12:18 ---------------------------------------------------
12 |    apply(() => f)
   |                  ^
   |                  No Manifest available for Unit.
1 error found

The first workaround makes it look like a syntactic parsing error, but the second workaround looks like a typing error during overload resolution.

[gaeljw]

For anyone coming here, with Scala 3 RC1 there is no more need for the implicit conversion to Fun0 thanks to the @targetName annotation and the following piece of code works as expected.

final class MyClass(name: String) {

  def apply(fun: => Any): Unit = {
    println("Got a By-name parameter")
  }

  @targetName("apply_fun0")
  def apply(fun: () => Any): Unit = {
    println("Got a Function0")
  }

  def apply[T1](f: T1 => Any): Unit = {
    println("Got a Function1")
  }

}

For instance, following code

MyClass("abc") {
  1
}

MyClass("abc") { () =>
  1
}

MyClass("abc") { (s: String) =>
  1
}

produces following output:

Got a By-name parameter
Got a Function0
Got a Function1