t7715.scala:8: error: error in interpolated string: identifier or block expected
  days map s"On the $_th day of Christmas" foreach println
                     ^
t7715.scala:10: error: error in interpolated string: identifier or block expected
  val rf = (n: Int) => s"\\*{$_}"(n).r
                              ^
t7715.scala:17: error: unbound placeholder parameter
  days zip days map s"${_: Int} by ${_: Int}".tupled foreach println
                        ^
t7715.scala:17: error: unbound placeholder parameter
  days zip days map s"${_: Int} by ${_: Int}".tupled foreach println
                                     ^
four errors found