デフォルト引数に指定する式には他の引数の値を利用することができない | Codetter（こーどったー）

scala> def log(msg:String, length:Int = msg.length) = "[%s] %s" format(length, msg)
<console>:5: error: not found: value msg
       def log(msg:String, length:Int = msg.length) = "[%s] %s" format(length, msg)
                                        ^

scala> def log(msg:String)(length:Int = msg.length) = "[%s] %s" format(length, msg)
log: (msg: String)(length: Int)String

scala> log("aaa bbb cccc")()
res18: String = [12] aaa bbb cccc