From: Richard Lawrence <rwl@...>
Date: 2007-07-12T01:10:00+09:00
Subject: Catching what a block returns (A newbie lesson learned)

Hello all,

This is a message intended to be from one newbie to another.

Until last night, I couldn't quite understand something about Ruby 
blocks:  namely, how to get anything back from them.  I have read a 
number of tutorials that try to explain blocks, and they always use 
trivial examples like this:

def verbose_call(x)
     puts "I am calling a block now!"
     yield x
     puts "I am done calling the block!"
end

and then you can do wonderful things like:

verbose_call(3) {|n| puts n + 1}
verbose_call("Jenny") {|name| puts name + " from the block."}

This is all well and good, but it seemed to me that this couldn't be all 
there was to the much-acclaimed power of blocks.  Then I saw things like:

[1, 2, 3].inject(4) {|sum, elt| sum + elt}  # => 10

and became confused.  Clearly, the value returned by the block is 
secretly being caught by inject(), then fed back into the block...but 
how?  This seemed a long way from the trivial examples:  there, blocks 
were a dead end, but here, there was two-way communication happening 
between the method and the block.

The solution (of course) was to realize that "yield" actually evaluates 
to something.  Thus,

def catch_block_value
     result = yield
     puts "I got this from the block: ", result
end

Realizing that "yield" evaluates to something I can assign to variables, 
etc. was important for getting beyond the trivial examples.  In 
retrospect, I should have paid more attention to the lesson that 
*everything* evaluates in Ruby.  I think what I was hung up on is that 
"yield" is a keyword (right? at least it looks like one), and hence I 
thought of it as mere flow control, not a passage for information.

So, again, totally trivial for all you veterans out there, but I hope 
that this clears some things up for someone like me.  Is there any 
documentation that I can help improve by doing a little write-up of this?

Richard