code.rb

class Code
  def initialize(code)
    @code = code
  end

  def process
    parse(transform(@code))
  end

  def transform(code)
    parts = code.split("|")
    chain = [parts.first]

    parts[1..-1].each_with_index do |m, i|
      _, meth, *args = m.strip.split(/^([^:]+):?/)
      args.unshift(chain.shift)
      chain << [meth, args]
    end

    chain.first
  end

  def parse(chain)
    meth, args = chain
    return meth unless args.respond_to?(:[])
    args[0] = parse(args[0]) if args[0].is_a?(Array)
    "#{meth}(#{args.join(', ')})"
  end
end

Code.new("'a,b' | split: ',' | first | upcase").process # upcase(first(split('a,b', ',')))
Code.new("'a' | link_to: 'www.terra.com.br'").process # link_to('a', 'www.terra.com.br')
Code.new("'b'").process # 'b'
Code.new("my_array | first | sum: 1").process # sum(first(my_array), 1)