在 Scala 中，如何在函数内声明静态数据? [英] In Scala, how would you declare static data inside a function?

问题描述

在许多情况下，我发现我需要在函数的作用域内创建长期存在的值，而这些数据不需要在类/对象作用域内.

In many situations I find that I need to create long-living values inside a function's scope, and there is no need for this data to be at class/object scope.

例如

object Example {

   def activeUsers = {
       val users = getUsersFromDB  // Connects to the database and runs a query.
       users.filter(_.active)
   }
}

上面，变量users在正确的范围内，但是每次调用activeUsers函数时它都会执行一次数据库查询.

Above, the variable users is in the correct scope, but it will execute a database query everytime the function activeUsers is called.

为了避免这种情况，我可以将变量 users 移到函数范围之外:

To avoid this, I could move the variable users outside the function's scope:

object Example {
   val users = getUsersFromDB  // Connects to the database and runs a query

   def activeUsers = {
       users.filter(_.active)
   }
}

但这也使其可用于其他功能.

But that makes it available to other functions as well.

否则，我可以创建一个单独的对象来封装函数:

Else, I could create a separate object to enclose the function:

object Example {

   object activeUsers {
       val users = getUsersFromDB  // Connects to the database and runs a query.

       def apply() = {
           users.filter(_.active)
       }
   }
}

但这涉及更多样板代码、使用另一个对象以及与 apply 相关的轻微语法奇怪.

But this involves more boilerplate code, use of another object and slight syntax oddities related to apply.

• 是否在语言层面支持这样的事情?
• 如果没有，您在这种情况下是否使用任何标准技术?

推荐答案

另一种选择是使用闭包:

Another option would be using a closure:

object Example {
   val activeUsers = {
       val users = getUsersFromDB
       () => users.filter(_.active)
   }
}

说明

activeUsers 是 Function1[Unit, ...your filter result type...] 类型的变量(或者我们可以将此类型写为 (Unit => ...你的过滤结果类型...)，这是一样的)，就是这个变量存储了一个函数.因此，您可以稍后以与函数无法区分的方式使用它，例如 activeUsers()

Explanation

activeUsers is a variable of type Function1[Unit, ...your filter result type...] (or we can write this type as (Unit => ...your filter result type...), which is the same), that is this variable stores a function. Thus you may use it later in a way indistinguishable from function, like activeUsers()

我们用一段代码初始化这个变量，我们在其中声明变量 users 并在匿名函数 () => 中使用它.users.filter(_.active)，因此它是一个闭包(因为它有一个绑定变量 users).

We initialize this variable with a block of code where we declare variable users and use it inside an anonymous function () => users.filter(_.active), hence it is a closure (as it has a bound variable users).

因此，我们实现了您的目标:(1) activeUsers 看起来像一个方法；(2) users 计算一次；(3) filter 适用于每次调用.

As a result, we achieve your goals: (1) activeUsers looks like a method; (2) users is calculated once; and (3) filter works on every call.

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