From: Thomas Hafner <thomas@...>
Date: 2009-05-02T22:40:04+09:00
Subject: lazy lookup

Hello,

how about the following implementation for lazy lookups?

  def define_lookup
    h = lambda{ |k|
      h = {}
      # ... fill h (expensive!) ...
      h[k]
    }
    self.class.class_eval{ define_method(:lookup){ |k| h[k] } }
  end

Once ``define_lookup'' has been called, the method ``lookup'' is
defined and will act in a lazy manner. Just call ``lookup'' as often
as you need, but only for the first time the expensive operation (e.g.
dealing with files) will be performed. Due to duck typing no condition
needs to be checked: for the first call ``h[k]'' implies calling the
lambda, for the next times ``h[k]'' implies accessing the hash.

(Tried with Ruby 1.8.6.)

Regards
  Thomas