tpl.rb

# -*- coding: utf-8 -*-

module Tpl
  def self.new_proc(template, variables = [])
    render_definition = "Proc.new do |__v, __o|\n __v ||= {}; __o ||= ''\n"
    compile(template, variables, render_definition)
    render_definition << "__o\nend"
    eval(render_definition)
  end

  def self.new_module(template, variables = [], m = Module.new)
    render_definition = "def self.render(__v = {}, __o = '')\n"
    compile(template, variables, render_definition)
    render_definition << "__o\nend"
    m.module_eval(render_definition)
    m
  end

  def self.compile(template, variables, render_definition)
    i = 0
    len = template.length

    render_definition << variables.map{|v| "#{v} = __v[:#{v}]\n"}.join

    loop do
      start = template.index("<%", i)

      unless start
        render_definition << "__o << #{template[i..len].inspect}\n"
        break
      end

      render_definition << "__o << #{template[i...start].inspect}\n"

      stop = template.index("%>", start)
      i = stop + 2

      case template[start + 2, 1]
      when "=" then
        render_definition << "__r = (#{template[start+3..stop-1]}).to_s\n__o << __r\n"
      when "#" then
        # ignore, # is a comment
      else
        render_definition << "#{template[start+3..stop-1]}\n"
      end
    end
  end
end

if $0 == __FILE__
  tpl = Tpl.new_proc("The sum is <%=2+2%> Also substitute <%=myfield%> ok", %w(myfield))
  puts tpl.call({:myfield => 10})
  tpl2 = Tpl.new_proc("ñañaña")
  puts tpl2.call
  tpl3 = Tpl.new_proc("Testing loops: <% 3.times do %> * <% end %>")
  puts tpl3.call

  class Testing
    def initialize(value)
      @variable = value
      tpl = Tpl.new_proc("@variable = <%= @variable %>")
      puts instance_exec(&tpl)
    end
  end

  Testing.new('some value')

  module Helper
    def the_helper
      "helper returned value"
    end
  end

  tpl4 = Tpl.new_module("Testing helper: <%= the_helper %>")
  tpl4.extend(Helper)
  puts tpl4.render

  puts Tpl.new_module('<% var = 10 %>var is <%= var %>').render
end