gistfile1.builder

class MyView < Mustache
  def cache_by_user_id
    lambda do |text|
      cache_key = "user:#{current_user.id}"

      if value = $cache.get(cache_key)
        value
      else
        rendered = render(text)
        $cache.set(cache_key, rendered)
        rendered
      end
    end
  end
end

# assumes $cache.set returns the value that is set, which seems sensible.
def $cache.fetch(key)
  get(key) || set(key, yield)
end

class MyView < Mustache
  def cache_by_user_id
    lambda { |text| $cache.fetch("user:#{current_user.id}"){ render(text) } }
  end
end

if $cache had a Hashlike interface, you could also do:

def cache_by_user_id
  lambda { |text| $cache["user:#{current_user.id}"] ||= render(text) }
end

The ||= short-circuits, preventing render(text) from being called if there's a cache hit.

Ruby is so expressive for this sort of problem.