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