require 'formula' class HgInstalled < Requirement def message; <<-EOS.undent Mercurial is required to use this software. You can install this with Homebrew using: brew install mercurial Or you can use an official installer from: http://mercurial.selenic.com/ EOS end def satisfied? which 'hg' end end class GitHg < Formula head 'https://github.com/offbytwo/git-hg.git' homepage 'http://offbytwo.com/git-hg/' depends_on HgInstalled.new def install prefix.install Dir['*'] end end