The template helpers serve to relieve the templates from including the same inline code again and again. It’s a set of standardized methods for working with forms (FormHelper), dates (DateHelper), texts (TextHelper), and Active Records (ActiveRecordHelper) that’s available to all templates by default.

It’s also really easy to make your own helpers and it’s much encouraged to keep the template files free from complicated logic. It’s even encouraged to bundle common compositions of methods from other helpers (often the common helpers) as they’re used by the specific application.

module MyHelper
  def hello_world() "hello world" end
end

MyHelper can now be included in a controller, like this:

class MyController < ActionController::Base
  helper :my_helper
end

…and, same as above, used in any template rendered from MyController, like this:

Let’s hear what the helper has to say: <%= hello_world %>

Methods
H
Instance Public methods
helper(*args, &block)

Declare a helper:

helper :foo

requires ‘foo_helper’ and includes FooHelper in the template class.

helper FooHelper

includes FooHelper in the template class.

helper { def foo() "#{bar} is the very best" end }

evaluates the block in the template class, adding method foo.

helper(:three, BlindHelper) { def mice() 'mice' end }

does all three.

Source: show 

# File rails/actionpack/lib/action_controller/helpers.rb, line 57
def helper(*args, &block)
  args.flatten.each do |arg|
    case arg
      when Module
        add_template_helper(arg)
      when String, Symbol
        file_name  = arg.to_s.underscore + '_helper'
        class_name = file_name.camelize
          
        begin
          require_dependency(file_name)
        rescue LoadError => load_error
          requiree = %r -- (.*?)(\.rb)?$/.match(load_error).to_a[1]
          msg = (requiree == file_name) ? "Missing helper file helpers/#{file_name}.rb" : "Can't load file: #{requiree}"
          raise LoadError.new(msg).copy_blame!(load_error)
        end

        add_template_helper(class_name.constantize)
      else
        raise ArgumentError, 'helper expects String, Symbol, or Module argument'
    end
  end

  # Evaluate block in template class if given.
  master_helper_module.module_eval(&block) if block_given?
end
helper_attr(*attrs)

Declare a controller attribute as a helper. For example,

helper_attr :name
attr_accessor :name

makes the name and name= controller methods available in the view. The is a convenience wrapper for helper_method.

Source: show 

# File rails/actionpack/lib/action_controller/helpers.rb, line 103
def helper_attr(*attrs)
  attrs.flatten.each { |attr| helper_method(attr, "#{attr}=") }
end
helper_method(*methods)

Declare a controller method as a helper. For example,

helper_method :link_to
def link_to(name, options) ... end

makes the link_to controller method available in the view.

Source: show 

# File rails/actionpack/lib/action_controller/helpers.rb, line 88
      def helper_method(*methods)
        methods.flatten.each do |method|
          master_helper_module.module_eval "            def #{method}(*args, &block)
              controller.send(%(#{method}), *args, &block)
            end
"
        end
      end