path.rb

require 'fileutils'
module RelativePaths extend self
  PREFIX_FILE="~/.babuska/cache/install_prefix".p
  PREFIX_ENV_VAR="BABUSHKA_PREFIX".freeze
  module Mixin
    # A helper for generating paths relative to a given path, specified in the
    # environment but cached from thenceforth

    def path(p)
      RelativePaths.prefix.p / p.p
    end
  end

  def prefix
    @prefix || get_prefix
  end

  def get_prefix
    prefix_from_env ||
      prefix_from_file ||
      "~"
  end

  def prefix_from_env
    if ENV.include? PREFIX_ENV_VAR
      write_prefix(ENV[PREFIX_ENV_VAR])
      ENV[PREFIX_ENV_VAR]
    end
  end

  def prefix_from_File
    File.read(PREFIX_FILE).chomp rescue nil
  end

  def write_prefix(p)
    FileUtils.mkdir_p(File.basename(PREFIX_FILE))
    File.open(PREFIX_FILE) do |f|
      f.write(p)
    end
  end
end

Object.send(:include, RelativePaths::Mixin)