引数の省略は後ろの引数から省略されていると解釈される | Codetter（こーどったー）

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scala> def date(month:Int=10, day:Int) = "%s/%s" format (month, day)
date: (month: Int,day: Int)String
scala> date(30)
<console>:7: error: not enough arguments for method date: (month: Int,day: Int)String.
Unspecified value parameter day.
       date(30)
           ^
scala> date(10,30)
res20: String = 10/30
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