comparing.rb

class Proc
  def on(g)
    f = self
    -> a, b { f[g[a], g[b]] }
  end
end

class Symbol
  def to_func
    -> this, *args { this.__send__ self, *args }
  end
end

module ProcUtils
  def comparing(&g)
    :<=>.to_func.on(g)
  end
end

if $0 == __FILE__
  include ProcUtils
  p [1,2,3,4].sort(&comparing(&:-@))
end