Scala是否具有等同于golangs defer的功能？

Question

Does Scala have an equivelent to golangs defer?

from: http://golang.org/doc/effective_go.html#defer

Go's defer statement schedules a function call (the deferred function) to be run immediately before the function executing the defer returns. It's an unusual but effective way to deal with situations such as resources that must be released regardless of which path a function takes to return. The canonical examples are unlocking a mutex or closing a file.

Answer 1

Scala does not offer defer by design, however you can create it yourself by wrapping your function in another function, passing an object which keeps track of functions to call.

Example:

class DeferTracker() {
  class LazyVal[A](val value:() => A)

  private var l = List[LazyVal[Any]]()
  def apply(f: => Any) = { l = new LazyVal(() => f) :: l }
  def makeCalls() = l.foreach { x => x.value() }
}

def Deferrable[A](context: DeferTracker => A) = {
  val dt = new DeferTracker()
  val res = context(dt)
  dt.makeCalls
  res
}

In this example, Deferable would be the wrapping function which calls context and returns it contents, giving it an object which tracks defer calls.

You can use this construct like this:

def dtest(x:Int) = println("dtest: " + x)

def someFunction(x:Int):Int = Deferrable { defer =>
  defer(dtest(x))
  println("before return")
  defer(dtest(2*x))

  x * 3
}

println(someFunction(3))

The output would be:

before return
dtest: 6
dtest: 3
3

I'm aware that this can be solved differently but it is really just an example that Scala supports the concept of defer without too much fuss.