Question

In Ruby, why can you write :

# b is not defined yet.
#
if b = true
  a = b
end
# => a = true

But not a one-liner :

a = b if b = true
# => NameError: undefined local variable or method `b' for main:Object

Answer 1

Because the Ruby interpreter “creates” a local variable when it sees an assignment.

In the second case, it hasn’t yet seen the assignment, so the variable doesn’t exist when the expression is parsed.

To be more precise, a method is first parsed into an internal representation, and then, perhaps, the code will eventually be called and actually executed.

Variables are “created” in that parsing pass. It’s really more a matter of declaration, it just means that the interpreter becomes aware of them. They won’t be created in the sense of being given space or a value until the surrounding method is called by someone.