From: Tom Cloyd <tomcloyd@...>
Date: 2009-01-15T11:00:54+09:00
Subject: Re: yield example baffling - need help

David A. Black wrote:
> Hi --
>
> On Thu, 15 Jan 2009, Tom Cloyd wrote:
>
>> Tom Cloyd wrote:
>>> I'm studying the yield statement for the first time, today.
>>>
>>> All's well until I see this example:
>>>
>>> def myeach(myarray)
>>> iter = 0
>>> while (iter < myarray.length):
>>> yield(myarray[iter])
>>> iter += 1
>>> end
>>> end
>>>
>>> testarray = [1,2,3,4,5]
>>> myeach(testarray) {|item| print "#{item}:"}
>>> → 1:2:3:4:5:
>>>
>>> I see a method with one parameter. There a block to the right of the 
>>> method call, inexplicably. It would make sense if some iteration 
>>> were going on, but I don't see that at all. One call, one execution 
>>> (with a loop), and we're out. What does "item" have to count through?
>>>
>>> How can that yield do anything? How does it know about the block? 
>>> It's not passed in. The iteration appears to be happening IN the 
>>> method, using a block it cannot possibly see. Ouch! After some 
>>> minutes, I'm not getting this at all.
>>>
>>> Can anyone clear my head for me. I've never seen anything like this. 
>>> Or is it just a weird Ruby idiom I'll just have to accept?
>>>
>>> Thanks in advance.
>>>
>>> Tom
>>>
>>>
>> Arrgh. What a dolt. I just realized that ALL the yield examples I've 
>> been studying have this block sitting over to the right - I was 
>> converting the curly braces to parens in my mind, and so didn't see 
>> them. It IS idiomatic weirdness. I yield to higher wisdom.
>
> Half the fun, actually, is that while you know what you're yielding,
> you never know exactly what you're yielding *to* :-)
>
> The block construct actually has two common use patterns: iterating
> through collections, and yielding "magic pen" objects.[1] An example
> of the magic pen is File.open:
>
> File.open("blah", "w") do |fh|
> fh.puts(whatever)
> end
>
> where the open method, which you never have to see, does the
> housekeeping, including closing the file for you. You just get the
> magic pen (the file handle). That pattern is, I think, particularly
> nice.
>
>
> David
>
> [1] http://dablog.rubypal.com/2007/2/18/the-magic-pens-of-ruby
>
David - very nice! Plus, you made me howl with laughter. Yes, I often 
cannot see to what I am yielding. And of course, we spend a lifetime 
approaching the Great Yield. Um...but not tonight, I think!

Thanks for your eloquence.

t.

-- 

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Tom Cloyd, MS MA, LMHC - Private practice Psychotherapist
Bellingham, Washington, U.S.A: (360) 920-1226
<< tc@tomcloyd.com >> (email)
<< TomCloyd.com >> (website) 
<< sleightmind.wordpress.com >> (mental health weblog)
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