What does map(&:name) mean in Ruby?

Question

I found this code in a RailsCast:

def tag_names
  @tag_names || tags.map(&:name).join(\' \')
end

What does the (&:name)

Answer 1

Josh Lee's answer is almost correct except that the equivalent Ruby code should have been as follows.

class Symbol
  def to_proc
    Proc.new do |receiver|
      receiver.send self
    end
  end
end

not

class Symbol
  def to_proc
    Proc.new do |obj, *args|
      obj.send self, *args
    end
  end
end

With this code, when print [[1,'a'],[2,'b'],[3,'c']].map(&:first) is executed, Ruby splits the first input [1,'a'] into 1 and 'a' to give obj 1 and args* 'a' to cause an error as Fixnum object 1 does not have the method self (which is :first).

---

When [[1,'a'],[2,'b'],[3,'c']].map(&:first) is executed;

1. :first is a Symbol object, so when &:first is given to a map method as a parameter, Symbol#to_proc is invoked.

2. map sends call message to :first.to_proc with parameter [1,'a'], e.g., :first.to_proc.call([1,'a']) is executed.

3. to_proc procedure in Symbol class sends a send message to an array object ([1,'a']) with parameter (:first), e.g., [1,'a'].send(:first) is executed.

4. iterates over the rest of the elements in [[1,'a'],[2,'b'],[3,'c']] object.

This is the same as executing [[1,'a'],[2,'b'],[3,'c']].map(|e| e.first) expression.