Scala：生成一个有条件地运行另一个块的块 [英] Scala: Generate a block that conditionally runs another block

问题描述

在 Circumflex框架中，您可以将URL映射到如下所示的块：

In the Circumflex framework, you can map an URL to a block like this:

get("/foo") = {
    "hello, world!"
}

当浏览到 / foo ，将按预期显示给定的字符串。现在，要编写完整的Web应用程序，几乎总是需要某种形式的身份验证和授权。我正在尝试为上述结构编写某种包装，所以我可以这样写：

which, when browsing to /foo, will show the given string as expected. Now, to write a complete web application, you almost always need some form of authentication and authorisation. I'm trying to write some kind of wrapper for the above construct, so I can write this:

get("/foo") = requireLogin {
    "hello, world!"
}

requireLogin 方法然后将检查用户是否已登录，如果是，则执行给定的块。但是，如果没有，它应该重定向到登录页面。

The requireLogin method would then check if the user is logged in, and if yes, execute the given block. If not, however, it should do a redirect to the login page.

现在我不知何故不能正确使用语法（我仍然是Scala新手）。

Now I somehow can't get the syntax right (i'm still a Scala newbie). How would you do this in a generic fashion?

推荐答案

尝试如下操作：

def executeMaybe[A](work: => A): Option[A] =
  if (util.Random.nextBoolean)
    Some(work)
  else
    None

这将以0.5的概率执行传递的代码，返回 Some（<工作交付的结果>）或返回无的情况是其他情况。您可以这样称呼它：

This executes the passed code with probability 0.5, returning Some(<result delivered by work>), or returns None is the other cases. You can call it either like this:

val v = executeMaybe(42)

或带有块符号：

val v = executeMaybe {
  // do some work
  // provide return value
}

诀窍是使用 by-name参数，该参数由 => 符号表示。阅读更多此处： http：//daily-scala.blogspot .com / 2009/12 / by-name-parameter-to-function.html

The trick is to use a by-name parameter, signalled by the => symbol. Read more e.g. here: http://daily-scala.blogspot.com/2009/12/by-name-parameter-to-function.html

这篇关于Scala：生成一个有条件地运行另一个块的块的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！