Question

Maybe I just don’t know enough about structs and have been using them blindly but the result below seems irrational to me.

class VarTest < Struct.new(:email)
  def perform
    puts "Start: #{email}"
    if email == "nothing"
      email = "bad email"
    end
    puts "End: #{email}"
  end
end
VarTest.new("[e-mail]").perform

Unexpected Output:

Start: [e-mail]
End:

If I change the code to:

class VarTest < Struct.new(:email)
  def perform
    e = email
    puts "Start: #{e}"
    if e == "nothing"
      e = "bad email"
    end
    puts "End: #{e}"
  end
end

VarTest.new("[e-mail]").perform

We get the expected output:

Start: [e-mail]
End: [e-mail]

Can someone please explain what is going on with this?

Thanks.

Answer 1

If you replace email = "bad email" with self.email = "bad email" it will work as expected. This is always true when using setters.

The reason is simple: when Ruby encounters a bareword, it tries to resolve it as a local var. If there is none, it will try to call a method by that name. Inside the class body self is the implicit receiver, so readers just work. Now for the writer there’s a problem. If you write something like foo = "bar", Ruby will create a new local variable, hence you need to make the receiver explicit.

This case is a bit trickier: if email == "nothing" uses the getter. However, email = "bad email" is still seen by the parser and a local variable email will be set to nil. This always happens when the parser sees a bareword as the LHS of an assignment. This local nil value is what makes it seem like the value of email disappears (which you can verify by only changing the last puts to puts "End: #{self.email}").