From: "David A. Black" <dblack@...>
Date: 2007-11-01T04:16:51+09:00
Subject: Re: .each do |foo, bar| what does bar do?

Hi --

On Thu, 1 Nov 2007, 7stud -- wrote:

> David A. Black wrote:
>> Hi --
>>
>> On Sun, 28 Oct 2007, 7stud -- wrote:
>>
>>>> class Hash
>>>>   def each
>>>>     each_key {|key| yield key, self[key] }
>>>>   end
>>>> end
>>>>
>>>
>>>
>>> That suffers the same problem as David Black's example.
>>
>> What problem did mine suffer from?
>>
>
> It doesn't return an array.

Was it supposed to?

>> def each(&block)
>>   orig_each(&block)
>>
>> versus
>>
>>>    orig_each(block)
>>>
>>> According to pickaxe2, p56, the '&' method converts the specified block
>>> to a Proc object and assigns it to the parameter variable 'block'.  Why
>>> is the second call to '&' required?
>>
>> Because there's a difference between passing a Proc around as an
>> object, and supplying a code block to a method. You can do both:
>>
>>    meth(arg,&block)
>>
>> and arg can be a Proc object. So there has to be some way to tell the
>> method what you're doing.
>>
>
> I'm not getting it.  With this definition:
>
> def each(&a_block)
>
> when I call:
>
> each() {some block}
>
> ruby converts the block to a Proc object and assigns it to the variable
> a_block.  So, it seems to me that after ruby passes the args specified
> in the method call to each(), the block would no longer be accessible
> inside the method--only the Proc object assigned to a_block would be
> accessible.  Are you saying that when the next line executes:
>
>> def each(&a_block)
>>   orig_each(&a_block)  <----****
>
> that the parameter variable a_block in the line:
>
>> orig_each(&a_block)
>
> is not the same variable as the a_block in the line:
>
>> def each(&a_block)

No, they're the same.

> ????  In other words, does a_block in the line:
>
>
>> orig_each(&a_block)
>
>
> reach outside the method definition and reference the block that is
> floating around in the ether?  Does ruby re-convert the block into a
> Proc object and re-assigns the Proc object to a_block?  If not, I don't
> understand why the second '&' is necessary: writing a_block should be
> enough to access the Proc object that ruby assigned to the parameter
> variable a_block when the method was first called.

It does access the Proc object. But accessing the Proc object is only
part of the story; there's also the question of what it does with the
Proc object.

If you do this:

   some_method(some_proc_object)

you're just passing the Proc object around the same way you would pass
a string or an array or any other object.

If you do this:

   some_method &some_proc_object

you're telling some_method that you want some_proc_object to play the
special role of code-block.

There could even be a situation where you would do:

   some_method(proc_1, proc_2) &proc_3

i.e., send two procs as regular arguments, and use a third one as the
code block.


David

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