Should Rails helpers assume an instance variable exists or should they receive them as parameters?

Question

I\'m wondering if there\'s a specific programming principle (Demeter?) that supports the idea that Rails helpers should never use controller instance variables, rather, they

Answer 1

I don't know if there is any named principle governing this sort of thing but I would pass an argument. Not only will the argument make your helper easier to test and your application's data flow easier to follow but it will also let you use one helper for a single instance as well as a list; if you pass an argument then both:

<%= cockadoodledoo @egg %>

and:

<% @eggs.each do |egg| %>
    <%= cockadoodledoo egg %>
<% end %>

will work as expected without introducing a special cockadoodledoo that handles a list in @eggs rather than a single @egg.