Ruby: WAT?

There’s a really fun video that went around a few days ago called Wat?. If you haven’t seen it yet, go do that now. I’ll wait..

One of the examples I find really fun(ny) is this one from Ruby:

1.9.2 :001 > a
NameError: undefined local variable or method `a' for main:Object
  from (irb):1
  from /usr/local/rvm/rubies/ruby-1.9.2-p290/bin/irb:16:in `<main>'
1.9.2 :002 > a = a
 => nil

Wat? :) Before I explain, let me do a few others I think are cool too (and are directly related). Each of these is executed in a fresh new IRB session. First, a simple one:

1.9.2 :001 > a = b
NameError: undefined local variable or method `b' for main:Object
  from (irb):1
  from /usr/local/rvm/rubies/ruby-1.9.2-p290/bin/irb:16:in `<main>'
1.9.2 :002 > a
 => nil # Wat..

Another I like:

1.9.2 :001 > c = 1 if c == 1
 => nil # WAT? No error?
1.9.2p290 :002 > c => nil # Wat?

A meaningful variation on the previous one:

1.9.2 :001 > if c == 1; c = 1; end
NameError: undefined local variable or method `c' for main:Object
  from (irb):1
  from /usr/local/rvm/rubies/ruby-1.9.2-p290/bin/irb:16:in `<main>'
1.9.2p290 :002 > c
 => nil # WAT?

And my personal favorite:

1.9.2 :001 > a = a.nil?
 => true

To explain, this is all due to how Ruby’s parser tears things apart. It actually creates a variable on the left hand side of an assignment before it evaluates the right hand side. Probably will never affect you, but definitely cool to know!

Ripper is really fun to dig around in, especially when having syntax problems or (more geekily) wanting to know how the ASTs are put together. Another post on that soon!