Ruby proc & symbol composition

comp.rb

def comp(*gs)
  id = -> x { x }
  gs.inject(id) do |f,g|
    -> x do
      f.to_proc.call g.to_proc.call x
    end
  end
end

times_2 = -> x { x * 2 }
plus_3 = -> x { x + 3 }

puts comp(times_2, plus_3).call(1) #=> 8
puts comp(plus_3, times_2).call(1) #=> 5

p (9..12).map(&comp(:reverse, :to_s)) #=> ["9", "01", "11", "21"]