From: 7stud -- <dolgun@...>
Date: 2007-09-24T13:52:25+09:00
Subject: Re: how to add an iterator to a class?

Here's a super simple example of an iterator:

def test
    yield
    yield
    yield

    puts "all done in test"
end


test {puts "hello"; sleep(2)}

In that example, test() calls the block but does not send it any 
arguments--just like you can call a method and not send it any 
arguments.  Note that the block is not defined to accept any arguments.

When the first yield statement is encountered, the block is called.  How 
does the yield statement know what block to call?  yield calls the block 
specified to the right of the test method call.  What if you call test 
and don't specify a block?

r4test.rb:2:in `test': no block given (LocalJumpError)
        from r4test.rb:9

After the first yield statement in test is encountered, execution halts 
at the point of the first yield statement until the block outputs 
"hello" and finishes sleeping.  Then execution continues in test.  The 
next line in test is another yield statement, so the block is called 
again, and execution halts in test until the block finishes.   The same 
thing happens a third time.  Finally, test outputs its terminating 
message.

Blocks can also return values to a yield statement--just like a method 
can return a value to a method call, e.g.


def get_calc
    10 * 2
end

def do_homework
    answer = get_calc   #return value replaces the method call in the 
code
    puts answer
end

do_homework


Here is how a block returns a value to a yield statement:

def test
    result = yield 10   #catch the value returned by the block in a 
variable

    puts result
end


test {10 * 2}
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