1.scala

scala> "a".format(_)
res0: Any* => String = <function1>

scala> "a".format(_: String)
res1: String => String = <function1>

scala> "a".format(_: String, _: String)
res2: (String, String) => String = <function2>
scala> "a".format(_: String, _: String, _: Int)
res3: (String, String, Int) => String = <function3>

scala> "a".format _
<console>:8: error: missing arguments for method format in trait StringLike;
follow this method with `_' if you want to treat it as a partially applied function
              "a".format _