brew/Library/Homebrew/formula_support.rb

# Used to track formulae that cannot be installed at the same time
FormulaConflict = Struct.new(:name, :reason)

# Used to annotate formulae that duplicate OS X provided software
# or cause conflicts when linked in.
class KegOnlyReason
  def initialize(reason, explanation)
    @reason = reason
    @explanation = explanation
  end

  def valid?
    case @reason
    when :provided_pre_mountain_lion
      MacOS.version < :mountain_lion
    when :provided_until_xcode43
      MacOS::Xcode.version < "4.3"
    when :provided_until_xcode5
      MacOS::Xcode.version < "5.0"
    else
      true
    end
  end

  def to_s
    return @explanation unless @explanation.empty?
    case @reason
    when :provided_by_osx then <<-EOS
OS X already provides this software and installing another version in
parallel can cause all kinds of trouble.
EOS
    when :shadowed_by_osx then <<-EOS
OS X provides similar software, and installing this software in
parallel can cause all kinds of trouble.
EOS
    when :provided_pre_mountain_lion then <<-EOS
OS X already provides this software in versions before Mountain Lion.
EOS
    when :provided_until_xcode43
      "Xcode provides this software prior to version 4.3."
    when :provided_until_xcode5
      "Xcode provides this software prior to version 5."
    else
      @reason
    end.strip
  end
end