function.rb

require 'binding_of_caller'

def self.method_missing(name, *args, &block)
  if binding.of_caller(1).eval("defined?(#{name})")
    binding.of_caller(1).eval("#{name}").call(*args, &block)
  else
    name
  end
end

def function(*param_names, &block)
  klass = Class.new { attr_accessor :this, *param_names }
  this  = TOPLEVEL_BINDING.eval('self')

  proc do |*params|
    context = klass.new
    context.this = this

    param_names.each do |param_name|
      context.send "#{param_name}=", params.shift
    end
    
    context.instance_eval &block
  end
end

fn = function(foo, bar) { foo + bar * 10; }

puts fn(1, 2) # => 21