From: Brian Candler <b.candler@...>
Date: 2009-01-16T22:22:18+09:00
Subject: Re: yield example baffling - need help

Tom Cloyd wrote:
> 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.

Maybe it's clearer if you rewrite myeach to show explicitly the block 
being passed in, and being called.

def myeach(myarray, &block)    # <<<
iter = 0
while (iter < myarray.length):
block.call(myarray[iter])      # <<<
iter += 1
end
end

Even if there is no &foo in the argument list, you can still pass a 
block when you call the method. The method can call it anonymously using 
'yield', and test for its presence using 'block_given?' (*)

BTW, Proc#[] is an alias for Proc#call, so this gives you three choices:

block.call(args)   #  only if block captured into a variable
block[args]        #  ditto
yield args

HTH,

Brian.

(*) It's occasionally a source of head-scratching when you pass a block 
to a method which doesn't expect one, and so it's silently ignored. Ruby 
can't and doesn't warn you of this.
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