为什么此Scala代码的编译错误类型不匹配 [英] Why this scala code has a compilation error type mismatch

问题描述

为什么使用此scala代码

Why this scala code

case class Foo[T]() {
  def bar(tt: T): Unit = ???
  def bar_(s: String)(implicit ev : T =:= String): Unit = bar(s)
}

触发此编译错误

[error] type mismatch;
[error]  found   : s.type (with underlying type String)
[error]  required: T
[error]     def foo2(s: String)(implicit ev: T =:= String) = foo(s)

推荐答案

问题是您需要证据 String =:= T 而不是 T =:= String

The thing is that you need evidence String =:= T rather than T =:= String.

case class Foo[T]() {
  def bar(tt: T): Unit = ???
  def bar_(s: String)(implicit ev : String =:= T): Unit = bar(s)
}

=:= 不对称.

https://typelevel.org/blog/2014/07/02/type_equality_to_leibniz.html

另请参见 cats.evidence.Is ， scalaz.Leibniz .

在scala 2.13中，您也可以这样做

In scala 2.13 you can also do

def bar_(s: String)(implicit ev : T =:= String): Unit = {
  implicit val ev1 = ev.flip
  bar(s)
}

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