What is the purpose of blocks?

Question

I've just started on ruby and can't wrap my head around blocks

How is it different from an anonymous function?

On what instance would I want to use it?

And when would I choose it over an anonymous function?

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.