semantics behind _ in Scala method calls

Question

scala 2.9.2 This compiles fine

object AppBuilder extends App {

  def app( blockw: Int => String ) : List[String] = List( blockw(6) )

  def app( block: => String ) : List[String] = app( _ => block )
}

But in REPL, the same methods/functions ( not sure about the distinction here) as above, when not enclosed in a class, I get the following errors

scala> def app( blockw: Int => String ) : List[String] = List( blockw(6) )
app: (blockw: Int => String)List[String]

scala> def app( block: => String ) : List[String] = app( _ => block )
<console>:8: error: missing parameter type
       def app( block: => String ) : List[String] = app( _ => block )
                                                     ^

score 5 · Accepted Answer · edited May 23 '17 at 12:27

Overloaded methods aren't currently supported in the REPL—see this answer for an explanation of why. You can prove this with a much simpler example:

scala> def f(x: Int) = x
f: (x: Int)Int

scala> def f(x: String) = x
f: (x: String)String

Now try f(0), and you'll get a type mismatch.

You can use paste mode as a workaround:

scala> :paste
// Entering paste mode (ctrl-D to finish)

def app( blockw: Int => String ) : List[String] = List( blockw(6) )
def app( block: => String ) : List[String] = app( _ => block )

// Exiting paste mode, now interpreting.

app: (blockw: Int => String)List[String] <and> (block: => String)List[String]
app: (blockw: Int => String)List[String] <and> (block: => String)List[String]

But you'd probably be better off avoiding overloading.

semantics behind _ in Scala method calls

1 Answers1