From: John Feminella <johnf@...>
Date: 2011-07-05T01:37:04+09:00
Subject: Re: Are blocks objects? What messages can they handle?

That is a fair point, Robert. I didn't use block_given? because I
wanted to show that indeed, `block` is an object by the time you can
see it in the method. Using "if block_given?" might not have made that
clear, but I thought that writing "if block" makes it unambiguous that
`block` is a real object with its own semantics.

~ jf
--
John Feminella
Principal Consultant, BitsBuilder
LI: http://www.linkedin.com/in/johnxf
SO: http://stackoverflow.com/users/75170/



On Mon, Jul 4, 2011 at 11:54, Robert Klemme <shortcutter@googlemail.com> wrote:
> On Sun, Jul 3, 2011 at 5:15 PM, Stephen Boesch <javadba@gmail.com> wrote:
>> John,
>>    although I was not original question author, wanted to thank you for
>> that great explanation and example.
>
> Just one point of nitpicking:
>
>> 2011/7/3 John Feminella <johnf@bitsbuilder.com>
>>
>>> A block is a syntactical construct, not an object. A Proc is the
>>> object-ized form of a block. If you want to do something on the block,
>>> you need to turn it into an object first.
>>>
>>> An analogy might be that the string "def" is a syntactical construct
>>> that creates a method, but it is not itself an object. You can't write
>>> "def.(...)" and have it be meaningful in this context.
>>>
>>> > So . . . how does one send a message to the block before it is passed as
>>> > an argument?  Is that even possible?
>>>
>>> You could make it a Proc first and pass that instead. For example, say
>>> you have a method which passes some arguments to a block:
>>>
>>> ======
>>> > def execute(*args, &block); yield *args if block; end
>>>  => nil
>
> I'd rather use these in pairs because the conversion of a block to an
> accessible object has some overhead:
>
> - block_given? and yield
> - &b and b[] / b.call
>
> So either
>
> def execute(*args)
>  yield *args if block_given?
> end
>
> or
>
> def execute(*args, &block)
>  block[*args] if block
> end
>
> respectively
>
> def execute(*args, &block)
>  block.call(*args) if block
> end
>
> Usually you only need the form with &block if you need to store the
> block or pass it on to another method, e.g.
>
> def foo
>  yield 123
> end
>
> def bar(&b)
>  foo(&b) # pass on
> end
>
> bar {|x| p x} # will print 123
>
> Kind regards
>
> robert
>
> --
> remember.guy do |as, often| as.you_can - without end
> http://blog.rubybestpractices.com/
>
>