gistfile1.rbx

# with :show_sign option and extended :format
def number_to_currency(number, options = {})
  options.symbolize_keys!

  defaults  = I18n.translate(:'number.format', :locale => options[:locale], :raise => true) rescue {}
  currency  = I18n.translate(:'number.currency.format', :locale => options[:locale], :raise => true) rescue {}
  defaults  = defaults.merge(currency)

  precision = options[:precision] || defaults[:precision]
  unit      = options[:unit]      || defaults[:unit]
  separator = options[:separator] || defaults[:separator]
  delimiter = options[:delimiter] || defaults[:delimiter]
  format    = options[:format]    || defaults[:format]
  show_sign = options[:show_sign] || defaults[:show_sign]

  separator = '' if precision == 0
  sign      = number < 0 ? '-' : '+'
  sign      = '' if (number < 0 && show_sign == :positive) || (number > 0 && show_sign == :negative)

  begin
    format.gsub(/%n/, number_with_precision(number.abs,
      :precision => precision,
      :delimiter => delimiter,
      :separator => separator)
    ).gsub(/%u/, unit).gsub(/%s/, sign)
  rescue
    number
  end
end

gistfile2.rbx

# with :zero_sign option
def number_to_currency(number, options = {})
  options.symbolize_keys!

  defaults  = I18n.translate(:'number.format', :locale => options[:locale], :raise => true) rescue {}
  currency  = I18n.translate(:'number.currency.format', :locale => options[:locale], :raise => true) rescue {}
  defaults  = defaults.merge(currency)

  precision = options[:precision] || defaults[:precision]
  unit      = options[:unit]      || defaults[:unit]
  separator = options[:separator] || defaults[:separator]
  delimiter = options[:delimiter] || defaults[:delimiter]
  format    = options[:format]    || defaults[:format]
  show_sign = options[:show_sign] || defaults[:show_sign]
  zero_sign = options[:zero_sign] || defaults[:zero_sign]

  separator = '' if precision == 0
  sign      = number == 0 ? zero_sign : (number < 0 ? '-' : '+')
  sign      = '' if (number < 0 && show_sign == :positive) || (number > 0 && show_sign == :negative)

  begin
    format.gsub(/%n/, number_with_precision(number.abs,
      :precision => precision,
      :delimiter => delimiter,
      :separator => separator)
    ).gsub(/%u/, unit).gsub(/%s/, sign)
  rescue
    number
  end
end