gistfile1.rb

def caseperm(str, num)
  str.each_char.map.with_index do |c,i|
    if num[i] == 0
      c.downcase
    else
      c.upcase
    end
  end.join
end

def caseperms(str)
  if block_given?
    (2**str.length).times do |i|
      yield caseperm(str, i)
    end
  else
    enum_for(__method__, str)
  end
end

caseperms("foo").to_a