CODEWITHSUNDEEP
ruby
planarian
planarian

Reputation: 2151

blocks and variable scope in Ruby

From p.226 of Russ Olsen's Eloquent Ruby:

All of the variables that are visible just before the opening do or { are still visible inside a code block. Code blocks drag along the scope in which they were created wherever they go.

To see this for myself, I ran the following code

class Foo
  def initialize
    @a = 1
  end

  def show_a
    puts "from method: value of a is #{@a}"
    yield
  end
end

bar = Foo.new

bar.show_a {puts "from block: value of a is #{@a}"}

The output, to my surprise, is

from method: value of a is 1

from block: value of a is

Why is the value of @a not making it into the block?

Upvotes: 0

Views: 74

Answers (1)

Chuck
Chuck

Reputation: 237060

Right before bar.show_a {puts "from block: value of a is #{@a}"}, stick in a puts @a. That will be nil as well. You will find that there is no @a instance variable visible in that scope, because @a is an instance variable of the Foo class. So the block is faithfully reflecting the lack of an @a in the surrounding scope.

To put it another way: A block closes over the scope where it's created, not where it's called.

Upvotes: 3

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