# No grok the &block

**URL:** <https://www.ruby-forum.com/t/no-grok-the-block/120252>\
**Category:** Ruby\
**Created:** [November 15, 2007, 7:28am UTC](https://www.ruby-forum.com/t/no-grok-the-block/120252 "2007-11-15T07:28:10Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [November 15, 2007, 7:28am UTC](https://www.ruby-forum.com/t/no-grok-the-block/120252/1 "2007-11-15T07:28:10Z")

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Trying to tackle Ruby bit by bit. Got many of the basic down, but I’m  
struggling to find an explanation for the significance and usefulness of  
&block. I think I understand that the important part is & and “block” is  
just a conventional name for the parameter, but as many times as I’ve  
seen &block used in places, I just haven’t been abe to recognize what it  
is doing (amid all the other reflective/dynamic stuff I’m still figuring  
out).

Anyone know of a good blog post that explains what it is/how it works,  
or care to tackle an explanation?

_much_ appreciated.

– gw

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<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [November 15, 2007, 8:06am UTC](https://www.ruby-forum.com/t/no-grok-the-block/120252/2 "2007-11-15T08:06:27Z")

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Greg W. wrote:

> Trying to tackle Ruby bit by bit. Got many of the basic down, but I’m  
> struggling to find an explanation for the significance and usefulness of  
> &block.

The best explanation I know is the one given by D.Black in his “Ruby for  
Rails” book, section 13.3, “Callable Objects”. It takes a bit of time to  
read it, but it is worth every line.

If you want a very short (and brutal) version:

1. blocks are those { … } that float in front of a method, to which  
the method yields.
2. when you want to access a block in a method, you need to list it in  
the arguments with an ‘&’; this lets Ruby know what this parameter  
means, but it also transforms the code block into a Proc object, that  
the method can call or pass to other methods. Eg:

def m1(&block); p block.class; block.call; end

m1 { puts “I am in a block” }

# =\> Proc

# =\> I am in a block

1. The story is not finished; if inside the method you want to pass the  
code block to another method, you need to prepend the & again, e.g. in a  
recursion over a binary tree:

def each(&block)  
@left.each(&block)  
block.call(self) # same as: yield  
@right.each(&block)  
end

We wrote @left.each(&block); why the ‘&’ again? to retransform the Proc  
into the code block that the child leaf will grab.

The essence of the story is: code blocks are NOT objects (there is no  
‘Block class’), so to be called they need to be transformed into objects  
(becoming a Proc); but they also need to go back to be themselves (when  
we pass them to somebody that expects a code block!). The ‘&’ does the  
trick during both trips!

This is a bit hard the first time you encounter it. But if you read that  
section of the book, and you practice, after a few days, it becomes  
second nature. Good luck

raul parolari