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frgomes/DesignByContract.scala

Created February 6, 2019 15:48
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DesignByContract.scala
def addNaturals(nats: List[Int]): Int = {
require(nats forall (_ >= 0), "List contains negative numbers")
nats.foldLeft(0)(_ + _)
} ensuring(_ >= 0)
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frgomes commented Feb 6, 2019

This is how ensuring is defined in Predef.scala:

  class Ensuring[A](x: A) {
    def ensuring(cond: Boolean): A = { assert(cond); x }
    def ensuring(cond: Boolean, msg: Any): A = { assert(cond, msg); x }
    def ensuring(cond: A => Boolean): A = { assert(cond(x)); x }
    def ensuring(cond: A => Boolean, msg: Any): A = { assert(cond(x), msg); x }
  }

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