procs 可以与 Ruby 2.0 中的 case 语句一起使用吗? [英] Can procs be used with case statements in Ruby 2.0?

问题描述

我记得在 Ruby 2.0 中的 case 语句中允许使用 procs，但我无法搜索它.

I remember something about procs being allowed in case statements in Ruby 2.0, but I can't google it.

我尝试查看 Ruby 2.0.0 NEWS 和 如何在 Ruby 中编写 switch 语句.我还访问了 http://ruby-doc.org ，但它的关键字链接是 Ruby 1.9，不是 Ruby 2.0.

I tried checking Ruby 2.0.0 NEWS and How to write a switch statement in Ruby. I also visited http://ruby-doc.org , but the link it had for keywords was for Ruby 1.9, not Ruby 2.0.

在 case 语句中是否允许使用 proc?

Are procs allowed in case statements?

推荐答案

是.

2.0.0p0 :001> lamb = ->(x){ x%2==1 }
#=> #<Proc:0x007fdd6a97dd90@(irb):1 (lambda)> 

2.0.0p0 :002> case 3; when lamb then p(:yay); end
:yay
#=> :yay 

2.0.0p0 :003> lamb === 3
#=> true 

2.0.0p0 :007> lamb === 2
#=> false 

然而，这与 1.9.1 没有什么不同，因为 Proc#=== 是当时定义的.由于 ruby​​-docs 似乎在显示此方法时有问题，要清楚文档说 proc === obj:

However, this is no different than 1.9.1 since Proc#=== was defined back then. Since ruby-docs seems to have a problem showing this method, to be clear the documentation says that proc === obj:

使用 obj 作为 proc 的参数调用块，如 #call.允许一个proc对象成为case语句中when子句的目标.

Invokes the block with obj as the proc's parameter like #call. It is to allow a proc object to be a target of when clause in a case statement.

<小时>

对于 Ruby 初学者，那么 Ruby 的 case 语句中的 when 子句获取子句中的值并调用 === 方法 on它，将参数传递给 case 语句.所以，例如，这段代码......

---

For the Ruby beginner, then when clause in Ruby's case statements takes the value in the clause and calls the === method on it, passing in the argument to the case statement. So, for example, this code…

case "cats"
  when /^cat/ then puts("line starts with cat!")
  when /^dog/ then puts("line starts with dog!")
end

...运行 /^cat/=== "cats" 来判断是否匹配；RegExp 类定义了 === 方法来执行正则表达式匹配.因此，你可以在 when 子句中使用你自己的对象，只要你为它定义了 ===.

…runs /^cat/ === "cats" to decide if it's a match; the RegExp class defines the === method to perform regex matching. Thus, you can use your own object in a when clause as long as you define === for it.

Moddable = Struct.new(:n) do
  def ===(numeric)
    numeric % n == 0
  end
end

mod4 = Moddable.new(4)
mod3 = Moddable.new(3)

12.times do |i|
  case i
    when mod4
      puts "#{i} is a multiple of 4!"
    when mod3
      puts "#{i} is a multiple of 3!"
  end
end

#=> 0 is a multiple of 4!
#=> 3 is a multiple of 3!
#=> 4 is a multiple of 4!
#=> 6 is a multiple of 3!
#=> 8 is a multiple of 4!
#=> 9 is a multiple of 3!

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