edit.haml

%h2
  Edit #{resource_name.to_s.humanize}
= form_for(resource, :as => resource_name, :url => registration_path(resource_name), :html => { :method => :put }) do |f|
  = devise_error_messages!
  %p
    = f.label :full_name
    %br
    = f.text_field :full_name
  %p
    = f.label :username
    %br
    = f.text_field :username
  %p
    = f.label :email
    %br
    = f.text_field :email
  %p
    = f.label :password
    %i (leave blank if you don't want to change it)
    %br
    = f.password_field :password
  %p
    = f.label :password_confirmation
    %br
    = f.password_field :password_confirmation
  -if @user.password_required?
    %p
      = f.label :current_password
      %i (we need your current password to confirm your changes)
      %br
      = f.password_field :current_password
  %p= f.submit "Update"
%h3 Cancel my account
%p
  Unhappy? #{link_to "Cancel my account", registration_path(resource_name), :confirm => "Are you sure?", :method => :delete}.
= link_to "Back", :back

new.haml

%h2 Sign up
= form_for(resource, :as => resource_name, :url => registration_path(resource_name)) do |f|
  = devise_error_messages!
  %p
    = f.label :email
    %br
    = f.text_field :email
  -if @user.password_required?
    %p
      = f.label :password
      %br
      = f.password_field :password
    %p
      = f.label :password_confirmation
      %br
      = f.password_field :password_confirmation
  %p= f.submit "Sign up"
= render :partial => "devise/shared/links"

user.rb

    # sign up with Twitter [works]
    #   redirect to /users/sign_up [works]
    #   it requires email [works]
    #   later, I
    #     I can update profile without a password [work, after adding 'update_with_password' method]
    #     I can add password (Password, Password confirmation) [works]
    #     After added password -> [current_password field should be required and visible) ->
    #       I should fill the current password in (current_password) field, to be able to update the profile

Class User
.
.
.
.
 def update_with_password(params={}) 
    if params[:password].blank? 
      params.delete(:password) 
      params.delete(:password_confirmation) if params[:password_confirmation].blank? 
    end 
    update_attributes(params) 
  end

  def password_required?
    (authentications.empty? || !password.blank?) && super
  end
.
.
.
.
.
end