ruby - 块的目的是什么？

Question

我刚开始使用红宝石，无法将头绕在积木上

它与匿名函数有何不同？

我想在什么情况下使用它？

我什么时候会选择它而不是匿名函数？

Answer 1

Ruby doesn't have anonymous functions like JavaScript (for example) has. Blocks have 3 basic uses:

1. Creating Procs
2. Creating lambdas
3. With functions

An example of where blocks are similar to anonymous functions is here (Ruby and JavaScript).

Ruby:

[1,2,3,4,5].each do |e| #do starts the block
  puts e
end #end ends it

JS (jQuery):

$.each([1,2,3,4,5], function(e) { //Anonymous *function* starts here
  console.log(e);
}); //Ends here

The power of Ruby blocks (and anonymous functions) is the fact that they can be passed to any method (including those you define). So if I want my own each method, here's how it could be done:

class Array
  def my_each
    i = 0
    while(i<self.length)
      yield self[i]
      i+=1
    end
  end
end

For example, when you declare a method like this:

def foo(&block)
end

block is a Proc object representing the block passed. So Proc.new, could look like this:

def Proc.new(&block)
  block
end

Blocks, by necessity, are bound to a method. They can only be turned into an object by a method like I described above. Although I'm not sure of the exact implementation of lambda (it does extra arg checking), but it is the same idea.

So the fundamental idea of a block is this: A block of code, bound to a method, that can either be contained in a Proc object by an & argument, or called by the yield keyword.