How is a local variable created even when IF condition evaluates to false in Ruby? [duplicate]

Question

Try the following in irb: (I'm using Ruby 2.0.0-p247)

blah
#=> NameError: undefined local variable or method `blah' for main:Object

if false
  blah = 'blah'
end
#=> nil 

blah
#=> nil

I'm surprised that blah is assigned nil even when the if condition evaluates to false. I thought the code within if is skipped as the condition evaluates to false.

Could someone with Ruby internals knowledge kindly explain how this happened?

Thank you

Answer 1

Local variables in ruby are created during parsing/compilation of code (not execution). They are lexically scoped, so a local variable is not visible before the line where it's assigned to.

defined?(foo) # => nil
if false
  defined?(foo) # => 
  foo = 'blah'
  defined?(foo) # => 
end

defined?(foo) # => "local-variable"
foo # => nil

defined?(foo) lines inside of if return nothing, because they didn't run. The assignment wasn't executed as well. However, the compiler saw the assignment to local variable and created one (with default value of nil).

This behaviour explains the trick from WAT talk:

a = a # => nil

Even though variable a doesn't exist, it is created (and set to nil) right before this line, simply because there is an assignment expression in the code (target of which is yet unknown local variable). So by the time the right hand side of this expression is evaluated, a exists.