Scala: error: forward reference extends over definition of value

Scala: Stream の再帰的な定義でエラー

 

自分自身を参照するような Stream を定義したら、エラーが出た。

 

エラーが発生する例

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scala> def f(x: Int) = {      |   val xs: Stream[Int] = x #:: xs.map(_ + 1)      |   xs.tail      | } :17: error: forward reference extends over definition of value xs          val xs: Stream[Int] = x #:: xs.map(_ + 1)                                      ^

 

対応方法

lazy val にすれば OK。

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scala> def f(x: Int) = {      |   lazy val xs: Stream[Int] = x #:: xs.map(_ + 1)      |   xs.tail      | } f: (x: Int)scala.collection.immutable.Stream[Int]   scala> f(3).take(10).toList res8: List[Int] = List(4, 5, 6, 7, 8, 9, 10, 11, 12, 13)

 

 

References

• scala - When does a Stream need to be lazy? - Stack Overflow