Question

scala> val input = readLine("hello %s%n", "world")
hello WrappedArray(world)
input: String = ""

scala> val input = Console.readLine("hello %s%n", "world")
hello world
input: String = ""

What’s the reason for the difference here? (I tried it compiled as well, so it’s not a REPL thing.)

Scala version 2.9.0-1

Answer 1

It seems like a bug in Predef:

def readLine(text: String, args: Any*) = Console.readLine(text, args)

When I think it should be:

def readLine(text: String, args: Any*) = Console.readLine(text, args: _*)

The first version you use is calling Prefef.readLine. Because of the missing _* type ascription, the function is called with args as the single first argument of the repeated argument args of Console.readLine.

In the uncurry compilation phase, this single argument is wrapped into a WrappedArray so that it it can be treated as a Seq[Any]. The WrappedArray is then converted using the toString method and this is what is used for %s in "hello %s%n". I think that is what happens.

In the second version args is treated from the start as a Seq[Any] and no conversion happens.

The whole thing is a bit funny, because in general the compiler does not let you do this:

scala> def f(s: Int*) = s foreach println
f: (s: Int*)Unit

scala> def g(s: Int*) = f(s)
<console>:8: error: type mismatch;
 found   : Int*
 required: Int
       def g(s: Int*) = f(s)

With Any, you get past the typer phase.