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

Two things are happening here, and it's important to understand both.

As described in other answers, the Symbol#to_proc method is being called.

But the reason to_proc is being called on the symbol is because it's being passed to map as a block argument. Placing & in front of an argument in a method call causes it to be passed this way. This is true for any Ruby method, not just map with symbols.

def some_method(*args, &block)
  puts "args: #{args.inspect}"
  puts "block: #{block.inspect}"
end

some_method(:whatever)
# args: [:whatever]
# block: nil

some_method(&:whatever)
# args: []
# block: #

some_method(&"whatever")
# TypeError: wrong argument type String (expected Proc)
# (String doesn't respond to #to_proc)

The Symbol gets converted to a Proc because it's passed in as a block. We can show this by trying to pass a proc to .map without the ampersand:

arr = %w(apple banana)
reverse_upcase = proc { |i| i.reverse.upcase }
reverse_upcase.is_a?(Proc)
=> true

arr.map(reverse_upcase)
# ArgumentError: wrong number of arguments (1 for 0)
# (map expects 0 positional arguments and one block argument)

arr.map(&reverse_upcase)
=> ["ELPPA", "ANANAB"]

Even though it doesn't need to be converted, the method won't know how to use it because it expects a block argument. Passing it with & gives .map the block it expects.