Question

类似于此Scala: Why does Seq.contains take an Any argument, instead of an argument of the sequence type?

如果您这样做 Seq(1, 2, 3).contains("dasdas")

这将编译并始终返回false。是否有引发类型错误的替代方法？这似乎是一个主要的wtf，其中contains总是返回false，并且在代码审查中非常容易错过。

Answer 1

您可以启用-Xfatal-warnings和-Ywarn-infer-any使其在类型不匹配时失败。
这也适用于相等性检查==。

以下是我在build.sbt中使用的示例，以避免像您遇到的那样奇怪的Scala：

scalacOptions ++= Seq(
  "-deprecation",
  "-explaintypes",
  "-feature",
  "-language:higherKinds",
  "-unchecked",
  "-Xcheckinit",
  "-Xfatal-warnings",
  "-Xfuture",
  "-Xlint",
  "-Yno-adapted-args",
  "-Ypartial-unification",
  "-Ywarn-dead-code",
  "-Ywarn-inaccessible",
  "-Ywarn-infer-any",
  "-Ywarn-nullary-override",
  "-Ywarn-nullary-unit",
  "-Ywarn-numeric-widen",
  "-Ywarn-unused"
) ++ (
  if (scalaVersion.value.startsWith("2.11")) Seq.empty
  else Seq("-Ywarn-extra-implicit")

Rob Norris的精彩文章涉及以下内容：
https://tpolecat.github.io/2017/04/25/scalac-flags.html

仅供参考：Scala 3中的通用相等将被通用相等取代，以解决您的问题：
http://dotty.epfl.ch/docs/reference/contextual/multiversal-equality.html

Answer 2

您可以编写扩展方法以仅接受定义的类型

implicit class ListOps[A](list: List[A]) {
  def typeSafeContains(a: A) = list.contains(a)
}

List(1, 2, 3).typeSafeContains(1)
List(1, 2, 3).typeSafeContains("does not work") //type mismatch error;

题外话，但我选中了haskell has typesafe contains

Prelude> elem 1 [1,2,3]
True

Prelude> elem "should not work" [1,2,3]

<interactive>:6:25: error:
    • No instance for (Num [Char]) arising from the literal ‘1’
    • In the expression: 1
      In the second argument of ‘elem’, namely ‘[1, 2, 3]’
      In the expression: elem "should not work" [1, 2, 3]

在类型不匹配时失败的`Seq.contains`的替代方案是什么？

2 个答案: