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object Main extends App {
 
  trait VirtualMachine[A] {
    def compile: A
    def run: A => Unit
  }
 
  case class SimpleVirtualMachine[A](compile: A, run: A => Unit)
      extends VirtualMachine[A]
 
  def helloWorldVM(s: String) = SimpleVirtualMachine(s, println)
  def intVM(i: Int) = SimpleVirtualMachine(i, println)
  def compileAndRun(vm: VirtualMachine[A forSome {type A}]) {
    vm.run(vm.compile)
  }
  compileAndRun(intVM(1))
}
 

Main.scala:16: error: type mismatch;
 found   : Main.SimpleVirtualMachine[Int]
 required: Main.VirtualMachine[A forSome { type A }]
Note: Int <: A forSome { type A }, but trait VirtualMachine is invariant in type A.
You may wish to define A as +A instead. (SLS 4.5)
  compileAndRun(intVM(1))
                     ^
one error found
spoj: The program compiled successfully, but Main.class was not found.
      Class Main should contain method: def main(args: Array[String]).

Standard output is empty