You can pass Ruby methods around; the syntax is just ugly, and it isn't really used:

    def call_on_two fn
      fn.call 2
    end

    call_on_two 1.method(:+)      #=> 3

The problem with [1, 2, 3].map(:function_name) is just that map requires a block, and not a method or proc.

A method like that could be easily enough created, though it'd be kind of ugly:

    module Enumerable
      def map_fn fn
        map { |i| method(fn).call i }
      end
    end

    def foo num
      num + 2
    end

    [1, 2, 3].map_fn :foo #=> [3, 4, 5]

I don't know why there isn't an easy way to freely convert between methods, procs, and blocks; it's definitely something the language is missing.