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

First, &:name is a shortcut for &:name.to_proc, where :name.to_proc returns a Proc (something that is similar, but not identical to a lambda) that when called with an object as (first) argument, calls the name method on that object.

Second, while & in def foo(&block) ... end converts a block passed to foo to a Proc, it does the opposite when applied to a Proc.

Thus, &:name.to_proc is a block that takes an object as argument and calls the name method on it, i. e. { |o| o.name }.