Scala中带有main()和extends App的对象的区别 [英] Difference between object with main() and extends App in scala

问题描述

我正在研究 ScalaInAction(书仍然是 MEAP，但代码在 github 上是公开的)现在我在第 2 章看这个 restClient::https:///github.com/nraychaudhuri/scalinaction/blob/master/chap02/RestClient.scala

首先，我使用 Scala 扩展设置了 intelliJ 并使用 main() 创建了一个 HelloWorld:

<所有导入>对象你好世界{def main(args: Array[String]) {<来自 RestClient.scala 的所有其余代码>}}

编译时出现以下错误:

scala:前向引用扩展值命令的定义val httppost = 新的 HttpPost(url)^

我可以通过移动以下几行来解决这个问题，直到相对于 def 的顺序是正确的

require( args.size >= 2, "你至少需要两个参数来进行获取、发布或删除请求")val 命令 = args.headval 参数 = parseArgs(args)val url = args.last命令匹配{案例发布"=>处理后请求案例获取"=>处理获取请求案例删除"=>处理删除请求案例选项"=>处理选项请求}

在浏览 github 页面时，我发现了这个:https://github.com/nraychaudhuri/scalinaction/tree/master/chap02/restclient

使用 extends App 而不是 main() 方法实现 RestClient.scala:

<所有导入>对象 RestClient 扩展应用程序 {<来自 RestClient.scala 的所有其余代码>}

然后我将我的 object HelloWorld 更改为仅使用 extends App 而不是实现 main() 方法并且它可以正常工作>

为什么执行此操作的 main() 方法会产生错误，而 extends App 不会?

解决方案

因为 main() 是一个方法，变量的 in 方法不能被前向引用.

例如:

对象测试{//x, y 是对象的实例变量，可以向前引用def sum = x + y//没问题值 y = 10值 x = 10}

但是方法中的代码无法向前引用.

对象测试{定义总和 = {val t = x + y//这不行，此时你没有 x, y值 x = 10值 y = 20val z = x + y//没问题}}

在您的情况下，如果您将所有代码从 RestClient.scala 复制粘贴到 main()，您将遇到同样的问题，因为 var url 在 handlePostRequest 中使用后声明.

I'm working through ScalaInAction (book is still a MEAP, but code is public on github) Right now I'm in chapter 2 looking at this restClient: : https://github.com/nraychaudhuri/scalainaction/blob/master/chap02/RestClient.scala

First, I setup intelliJ with scala extensions and created a HelloWorld with main():

<ALL the imports>

object HelloWorld {
   def main(args: Array[String]) {
     <ALL the rest code from RestClient.scala>
   }
}

I get the following error when compiling:

scala: forward reference extends over defintion of value command
val httppost = new HttpPost(url)
                ^

I can fix this by moving the following lines around until the ordering is correct with relation to the def's

require( args.size >= 2, "You need at least two arguments to make a get, post, or delete request")

val command = args.head
val params = parseArgs(args)
val url = args.last

command match {
  case "post"    => handlePostRequest
  case "get"     => handleGetRequest
  case "delete"  => handleDeleteRequest
  case "options" => handleOptionsRequest
}

While browsing the github page, I found this: https://github.com/nraychaudhuri/scalainaction/tree/master/chap02/restclient

Which uses implements RestClient.scala using extends App instead of a main() method:

<All the imports>
object RestClient extends App {
   <All the rest of the code from RestClient.scala>
}

I then changed my object HelloWorld to just use extends App instead of implementing a main() method and it works without errors

Why does the main() method way of doing this generate the error but the extends App does not?

解决方案

Because main() is a method, and variable's in method could not be forward reference.

For example:

object Test {

   // x, y is object's instance variable, it could be forward referenced

   def sum = x + y // This is ok
   val y = 10    
   val x = 10

}

But code in a method could not forward referenced.

object Test {
    def sum = {
        val t = x + y // This is not ok, you don't have x, y at this point
        val x = 10
        val y = 20
        val z = x + y // This is ok
    }
}

In your case, if you copy paste all codes from RestClient.scala to main(), you will have the same issue because var url is declared after its usage in handlePostRequest.

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