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Block Scope

...or, how closures break encapsulation (in a good way)

Blocks see the variables of their definer, not their caller

Example of Block Scope

def twice
  yield
  yield
end

def flatter
  message = "you are great"
  twice do
    puts "#{message}!"
  end
end

message is in the scope of flatter, not twice

locals, function parameters, block parameters

def twice_with word
  yield word
  yield word.upcase
end

def flatter person
  message = "you are great"
  twice_with(person) do |name|
    puts "#{message}, #{name}!"
  end
end

flatter "Alex"
  • person is a parameter of flatter
  • message is a local variable of flatter
  • name is a parameter of the block
  • word is a parameter of twice_with

Q: Which variables are available inside the block?

Nested Scopes

  • Every function call creates a new parent scope
  • A block inside that function creates a child scope
  • Variable lookup proceeds up the scope chain

With Great Power Comes Great Responsibility

  • Closures drag data along with them
    • Sometimes they drag more than you intended
  • Possible dangers:
    • memory leaks
    • side effects
    • namespace collision
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