Scala REPL中的嵌套环境 [英] Nested environments in Scala REPL

问题描述

是否有可能在Scala REPL中创建( enter )嵌套环境，这样在退出嵌套环境之后，在已存在环境中创建的所有变量绑定都将丢失. ?

Is it possible to create (enter) nested environments in Scala REPL, such that after exiting the nested environment, all variable bindings created within the exited environment will be lost?

这是我希望一个会话的样子:

Here is what I wish a session could look like:

scala> val x = 1
x: Int = 1

scala> enter // How to implement this?
// Entering nested context (type exit to exit)

scala> val x = 2
x: Int = 2

scala> val y = 3
y: Int = 3

scala> exit // How to implement this?
// Exiting nested context

scala> assert(x == 1)

scala> y
<console>:12: error: not found: value y
       y
       ^

scala> 

推荐答案

当前的Scala REPL无法做到这一点，但是您可以使用

This isn't possible with the current Scala REPL, but you can achieve something similar using the Ammonite REPL:

Welcome to the Ammonite Repl 0.8.2
(Scala 2.12.1 Java 1.8.0_121)
@ val x = 1 
x: Int = 1
@ repl.sess.save("first")
res1_1: ammonite.repl.SessionChanged = 
@ val x = 2 
x: Int = 2
@ val y = 3 
y: Int = 3
@ repl.sess.save("second") ; repl.sess.load("first") 
res4_1: ammonite.repl.SessionChanged = 
Removed Imports: Set('y, 'res1_1, 'res1_0)
@ y 
cmd5.sc:1: not found: value y
val res5 = y
           ^
Compilation Failed
@ x 
res5: Int = 1

这些会话并不完全按照您的描述嵌套，但是很容易按名称进行跟踪，并且可以重叠.也就是说，在repl.sess.save("first")之后，如果不覆盖原始的x，您仍然可以访问它.

These sessions aren't nested exactly the way you describe, but are easy to track by name, and can overlap. That is after repl.sess.save("first"), you still have access to the original x if you don't override it.

玩了更多之后，我得以构筑了一个简单的对象，该对象使用堆栈来跟踪会话并加载/保存它们.可以将其放置在~/.ammonite/predef.sc中，以使用Ammonite REPL自动加载:

After playing around with it some more, I was able to concoct a simple object that uses a stack to track the sessions and load/save them. It can be placed in ~/.ammonite/predef.sc to load automatically with the Ammonite REPL:

object SessionStack {

    case class AmmSession(id: Int = 1) {
        def name = s"session_${id}"
        def next = AmmSession(id + 1)
    }

    private var sessions = collection.mutable.Stack.empty[AmmSession]

    private var current = AmmSession()

    def enter: Unit = {
        sessions.push(current.copy())
        repl.sess.save(current.name)
        current = current.next
    }

    def exit: Unit = if(sessions.nonEmpty) {
        current = sessions.pop()
        repl.sess.load(current.name)
    } else {
        println("Nothing to exit.")
    }

}
import SessionStack._

我没有对此进行严格的测试，因此可能有一个未覆盖的边缘保护套，但是我能够轻松地深入几层然后将其剥离.

I haven't tested this rigorously, so there may be an edge-case that isn't covered, but I was able to go a few levels deep easily and then peel back the layers.

这篇关于Scala REPL中的嵌套环境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！