From: Florian Gross <flgr@...>
Date: 2004-09-18T19:24:44+09:00
Subject: Re: Accessing Hash elements in sorted order?

ChrisO wrote:

> When I saw the parens, I thought they might need to be there, but I 
> digressed from asking since it was simple enough in irb to experiment 
> and find out they were NOT needed.
> 
> Still, it initially lead me as a fairly new player in Ruby, to consider 
> that the parens were perhaps some "mapping" mechanism since I had a 
> single item "pair" in that place before.  Instead, when I realized the 
> parens were superfluous, I reasoned that Ruby was smart enough to "map" 
> the key/value on it's own it would seem.  (Similar to #scan on Match 
> objects from the #scan regex?  Kinda?  If someone would care to 
> comment...?)

It's all just Ruby's assignment semantics at work. I'm not sure if 
they're documented in the old Pickaxe at all, but if they are the 
documentation will be slightly out-dated. (Semantics were different in 
1.6.8 and I think that between 1.8.0 and 1.8.1 they changed slightly for 
block parameters in respect to the difference between lambda { } and 
Proc.new)

You can see the connections here:

a = [1, 2] # a: [1, 2]
a, = [1, 2] # a: 1
a = 1, 2 # a: [1, 2] -- equivalent to yield() sample giving warning
a, b = [1, 2] # a: 1, b: 2
(a, b) = [1, 2] # a: 1, b: 2 -- same as a, b = *[1, 2]

eval("yield [1, 2]") { |a| p a } # a: [1, 2]
eval("yield [1, 2]") { |a,| p a } # a: 1
eval("yield [1, 2]") { |a,b| p [a,b] } # a: 1, b: 2
eval("yield [1, 2]") { |(a,b)| p [a,b] } # a: 1, b: 2

eval("yield(1, 2)") { |a| p a } # a: [1, 2] -- warning
eval("yield(1, 2)") { |a,| p a } # a: 1
eval("yield(1, 2)") { |a,b| p [a,b] } # a: 1, b: 2
eval("yield(1, 2)") { |(a,b)| p [a,b] } # a: 1, b: 2

Regards,
Florian Gross