ISO4217 number_to_currency Proof of Concept

poc.rb

  def number_to_currency(number, options = {})
    if options[:currency_code].present?
      options[:unit] = unit_from_currency_code(options[:currency_code])
    end

    super
  end

  private

  def unit_from_currency_code(code)
    code = code.downcase.to_sym unless code.is_a? Symbol

    unit = case code
            when :gbp
              "£"
            else
              "$"
            end

    return unit
  end

poc_spec.rb

  describe '#number_to_currency' do
    context 'with currency_code option' do
      context 'as a symbol' do
        it 'returns the number with the correct currency' do
          value = number_to_currency(10.5, currency_code: :gbp)
          expect(value).to eq "£10.50"
        end
      end

      context 'as a lower-case String' do
        it 'returns the number with the correct currency' do
          value = number_to_currency(10.5, currency_code: 'gbp')
          expect(value).to eq "£10.50"
        end
      end

      context 'as an upper-case String' do
        it 'returns the number with the correct currency' do
          value = number_to_currency(10.5, currency_code: 'GBP')
          expect(value).to eq "£10.50"
        end
      end
    end
  end