From: "Hal E. Fulton" <hal9000@...>
Date: 2002-09-23T22:09:43+09:00
Subject: Re: Teach me about yield

----- Original Message ----- 
From: "Joseph Benik" <jobeicus@hotmail.com>
Newsgroups: comp.lang.ruby
To: "ruby-talk ML" <ruby-talk@ruby-lang.org>
Sent: Monday, September 23, 2002 7:15 AM
Subject: Teach me about yield


> i've programmed computers for the greater portion of my life and find
> myself really "set in my ways" when it comes to solving problems.  i
> have both Dave and Matz's ruby books and have read the description of
> yield numerous times, yet never used a single yield statement in just
> over a year of ruby coding.
> 
> when do you use yield?  has it solved problems for you when you
> couldn't find another solution?  how did you first start using yield?

Here's a way to look at it.

Do you find "each" useful? And other iterators?

Have you ever wanted to write an iterator for a class
of your own (that didn't inherit from a class already
having one)?

In a case like that, you would use yield.

Have you ever wanted to write (what I call) a "non-iterating
iterator" that does some housekeeping, calls a block, and
does more housekeeping? Then you would use yield.

Definitive examples of the non-iterating iterator:
  File.open("file") { block }  # open, do block, close
  Mutex.synchronize { block }  # grab, do block, release
  Dir.chdir("dir")  { block }  # cd, do block, cd back  
 
If you don't understand how this works, try this:

def do_something
  puts "Before"
  yield  # Basically call the block
  puts "After"
end

do_something { puts "I'm doing something" }

Output:
  Before
  I'm doing something
  After

Secondly, note that if you want an "iterating" iterator, you
would do a yield inside a loop. Control is traded back and
forth between the iterator and its block.

Thirdly, note that yield returns a value (which is the last
expression evaluated inside the block). This is useful for
things like Enumerable#select and others.

> i want to expand my mind and wrap it around this concept, any pointers
> (besides RTFM), or sample code would be greatly appreciated

There's nothing wrong with RTFM (Ruby Training From Matz).  :)


HTH,
Hal