External command: brew switch formula version

This external command allows you to switch between installed versions of
a formula. If you have multiple versions of a formula in your cellar, the
standard "brew link" command will refuse to run.

This new command "brew switch" tries to unlink all versions of the brew
from the prefix, then links the specific requested version.

Library/Contributions/examples/brew-switch.rb

@@ -0,0 +1,40 @@
+require 'formula'
+require 'keg'
+
+if ARGV.named.length != 2
+  onoe "Two parameters are expected, the formula name and version to link."
+  exit 1
+end
+
+name = ARGV.shift
+version = ARGV.shift
+
+# Does this formula have any versions?
+f = Formula.factory(name.downcase)
+cellar = f.prefix.parent
+unless cellar.directory?
+  onoe "#{name} not found in the Cellar."
+  exit 2
+end
+
+# Does the target version exist?
+unless (cellar+version).directory?
+  onoe "#{name} does not have a version #{version} in the Cellar."
+
+  versions = cellar.children.select { |pn| pn.directory? }.collect { |pn| pn.basename.to_s }
+  puts "Versions available: #{versions.join(', ')}"
+
+  exit 3
+end
+
+# Unlink all existing versions
+cellar.children.select { |pn| pn.directory? }.each do |v|
+  keg = Keg.new(v)
+  puts "Cleaning #{keg}"
+  keg.unlink
+end
+
+# Link new version
+
+keg = Keg.new(cellar+version)
+puts "#{keg.link} links created for #{keg}"