$ scala
Welcome to Scala version 2.10.0 (Java HotSpot(TM) 64-Bit Server VM, Java 1.7.0_15).
Type in expressions to have them evaluated.
Type :help for more information.

scala> def f(a1: Int, a2: Int, a3: Int, a4: Int, a5: Int, a6: Int, a7: Int, a8: Int, a9: Int, a10: Int, a11: Int, a12: Int, a13: Int, a14: Int, a15: Int, a16: Int, a17: Int, a18: Int, a19: Int, a20: Int, a21: Int, a22: Int, a23: Int) = 0
f: (a1: Int, a2: Int, a3: Int, a4: Int, a5: Int, a6: Int, a7: Int, a8: Int, a9: Int, a10: Int, a11: Int, a12: Int, a13: Int, a14: Int, a15: Int, a16: Int, a17: Int, a18: Int, a19: Int, a20: Int, a21: Int, a22: Int, a23: Int)Int

scala> (f _)
<console>:9: error: missing arguments for method f;
follow this method with `_' if you want to treat it as a partially applied function
              (f _)
               ^