How [NOT] to require external dependencies

not_so_flexible_require

#
# This method allows requiring dependencies with some flexibility.
#
# Implemented algorithm makes greedy choices about the environment:
# 1. It first attempts a simple <code>Kernel.require(name)</code> before 
#    anything else (even bypassing version requirement)
# 2. If step 1 fails with a LoadError then it falls back requiring the 
#    gem with specified version (defaults to >= 0) and retries step 1.
# 3. If step 2 fails with a NameError, 'rubygems' are required and step 
#    2 is retried.
# 4. If step 3. fails, the initial LoadError is reraised.
#
# Doing so ensures flexibility for the users of the library by not making
# wrong assumptions about their environment. Testing the library is also
# made easier, as illustrated in the examples below. Please note that this
# method is useful to load external dependencies of your code only, not 
# .rb files of your own library.
#
# Examples:
#
#   # Require something from the standard library
#   Noe::Loader.require('fileutils')
#
#   # Require a gem without specifing any particular version
#   Noe::Loader.require('highline')
#
#   # Require a gem, specifing a particular version
#   Noe::Loader.require('foo', "~> 1.6")
#
#   # Twist the load path to use version of foo you've recently 
#   # forked (bypass the version requirement)
#   $LOAD_PATH.unshift ...   # or ruby -I...
#   Noe::Loader.require('highline', "~> 1.6")
#
# Learn more about this pattern:
# - http://weblog.rubyonrails.org/2009/9/1/gem-packaging-best-practices
# - https://gist.github.com/54177
#
def not_so_flexible_require(name, version = nil)
  Kernel.require name.to_s
rescue LoadError
  begin
    gem name.to_s, version || ">= 0"
  rescue NameError
    if $VERBOSE
      Kernel.warn "#{__FILE__}:#{__LINE__}: warning: requiring rubygems myself, "\
                  " you should use 'ruby -rubygems' instead. "\
                  "See https://gist.github.com/54177"
    end
    require "rubygems"
    gem name.to_s, version || ">= 0"
  end
  Kernel.require name.to_s
end