kotlin合约的目的是什么

Question

正在阅读apply函数代码源码，发现

contract {
        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    }

并且合约有一个空的主体，实验性的

@ContractsDsl
@ExperimentalContracts
@InlineOnly
@SinceKotlin("1.3")
@Suppress("UNUSED_PARAMETER")
public inline fun contract(builder: ContractBuilder.() -> Unit) { }

合同的真正目的是什么，是否会保留在下一个版本中？

Answer 1

合同的真正目的是什么

Kotlin合约的真正目的是帮助编译器做出一些自己无法做出的假设。有时，开发人员比编译器更了解某个功能的用法，并且可以将特定的用法教给编译器。

callsInPlace既然你提到了，我就举个例子。

想象一下有以下功能：

fun executeOnce(block: () -> Unit) {
  block()
}

并以这种方式调用它：

fun caller() {
  val value: String 
  executeOnce {
      // It doesn't compile since the compiler doesn't know that the lambda 
      // will be executed once and the reassignment of a val is forbidden.
      value = "dummy-string"
  }
}

Kotlin合约在这里有帮助。您可以callsInPlace用来告诉编译器该 lambda 将被调用多少次。

@OptIn(ExperimentalContracts::class)
fun executeOnce(block: ()-> Unit) {
    contract {
        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    }
    block()
}

@OptIn(ExperimentalContracts::class)
fun caller() {
  val value: String 
  executeOnce {
      // Compiles since the val will be assigned once.
      value = "dummy-string"
  }
}

它会保留在下一个版本中吗？

Who knows. They are still experimental after one year, which is normal for a major feature. You can't be 100% sure they will be out of experimental, but since they are useful and they are here since one year, in my opinion, likely they'll go out of experimental.