rate_converter.rb

#! /usr/bin/env ruby

require 'json'
require 'net/http'
require 'uri'

def rate_converter(from, to)
  uri = URI('http://rate-exchange.appspot.com/currency')
  uri.query = URI.encode_www_form(from: from, to: to)

  JSON.parse(Net::HTTP.get(uri))['rate']
end

rate_converter('USD', 'CHF')              # => 0.969600127
rate_converter('USD', 'GBP')              # => 0.662295516