呼叫/cc和“确保"的语义包括:在Ruby中 [英] The semantic of call/cc and "ensure" in Ruby

问题描述

到目前为止，我知道Ruby是唯一同时支持call/cc和try/catch/finally(写为begin/rescue/ensure/end块)的主流语言.

As I know so far, Ruby is the only mainstream language that supports both call/cc and try/catch/finally (written as begin/rescue/ensure/end block).

我不熟悉Ruby，但是我的直觉告诉我这两者之间可能存在冲突，因为call/cc允许任意控制流，而ensure则需要一定的控制流(某些代码路径必须在A中执行预定义的情况，即离开包含块).

I am not familiar with Ruby, but my intuitive tell me that there are potential conflicts of that two, since call/cc allows arbitrarily control flow and ensure require some guaranteed control flow (some code path MUST be executed in a pre-defined situation, namely leaving the containing block).

那么，该语言是否存在任何冲突?如果是这样，在这种情况下该语言的定义行为是什么?尤其是，如果在begin/ensure块或ensure/end块中使用call/cc，会发生什么情况?如果在包含ensure子句的块后调用捕获的call/cc怎么办?

So, are there any conflicts exists in the language? if so what is the defined behavior of the language in such a case? Especially, what happen if call/cc being used in a begin/ensure block or in the ensure/end block? What if calling a captured call/cc after the block contains ensure clause?

推荐答案

在Ruby中，您有 和 raise /rescue/ensure.

In Ruby you have callcc, throw / catch and raise / rescue / ensure.

throw通过开始执行ensure终止该块:

throw terminates the block with ensure begin executed:

catch(:done) {
  begin
    puts "before"
    throw :done
    puts "after"  # will not be called
  ensure
    puts "ensure"
  end
}

产生:

before
ensure

---

callcc以ensure开始跳过来终止该块:

---

callcc terminates the block with ensure begin skipped:

callcc { |cc|
  begin
    puts "before"
    cc.call
    puts "after"   # will not be called
  ensure
    puts "ensure"  # will not be called
  end
}

产生:

before

---

另一个将 Continuation 对象存储在全局变量中的示例:

---

Another example storing the Continuation object inside a global variable:

begin
  puts "before"
  callcc { |cc| $cc = cc }
  puts "after"
ensure
  puts "ensure"
end

$cc.call

产生:

before
after
ensure
after
ensure
after
...

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